Other later bladed weapons though made for fighting much lighter armored opponents would have been much easier to hit gaps in the armor and unless it is a full suit there will be lots of unfortunate gaps, though finding these could go up with skill.
Are you referring to the eventual shift from heavier arming swords and long swords to the rise of later small swords and of the more commonly known rapier?
If so... then it is not exactly the need for faster blades to find openings that lead to lighter blades. In fact, if you were to match a rapier against a longsword or an arming sword, in an even match, the rapier would likely break long before either of the other blades and would be at quite a disadvantage against a heavily armored opponent.
A lighter blade possesses less mass, and therefore possesses less force, meaning it has less penetrating power than larger swords. Because of this lack of power, they were generally used primarily as cutting or thrusting weapons against unarmored foes. Due to their smaller masses, they were also more susceptible to being damaged by a larger, more powerful weapon. They were mostly useless against armored foes. Sure you could still try to find openings and weak spots, but against a heavily armored opponent who either has a sword/shield (far more defense than you), a longsword (far more power and more range), or a spear, you are horribly outmatched. A foe isn't going to simply stand around as you try to poke him in a weak spot. You have a better chance of closing in on an opponent and stabbing them in the armpit with a dagger than you do of poking him from afar where he can easily bat away an attack. Even closing in and trying to stab someone accurately in the armpit (or other weak point) would be difficult from close range when wielding a weapon approximately 3 feet in length.
Against an experienced sword/shield user in heavy armor, you're not going to get any easy openings. It is not much, if at all, easier to find openings with a lighter blade than with a heavier one. In fact it puts you at a greater disadvantage when pitted against a heavily armed foe. One of you is either tiring out first, or getting knocked down. You're not going to have the time to carefully aim a shot to a weak spot reliably like that. Unless you stun him with a good blow to the head or knock him down, or if he overextended too far. Even then, with a lighter weapon, you have less power, and may not be able to adequately stun or disorient him with a shot to a heavy helmet. You have a split second to go for an opening usually, and you can do that with a heavy sword as well as a lighter sword. You'd be surprised how easy it is to twist a hand-and-a-half sword upwards and thrust (the leverage action of having two hands makes it amazingly easy), even with a heavy blade. Even with a split second opening, you're not guaranteed a hit to a weak spot, as your opponent will likely react and defend themselves.
Lighter weapons were developed as armor got lighter. Less armor, you needed less force to kill your opponents. Why carry around overly heavy, tiring weapons when you can carry around lighter, faster blades? Plus you could use the extra speed to your advantage as with less armor, you become more mobile again. The primary cause for armor becoming lighter was the development and advancement of firearm technologies. As guns and cannons became more widely used, more accurate, and faster, they were used more often. Heavy armor could not withstand gun fire, and sitting in a field trying to run away from gun fire in heavy armor was an easy way to get shot. So equipment became lighter in response to new technologies, and sidearms became lighter as well, due to the response of lighter armors.
If you really want to insist that an unarmored opponent with a rapier has an advantage over a heavy armored foe, think of it in a manner similar to this: Jeep vs. Tank.
Sure you can run circles around the heavily armored foe, but unlike a jeep, you risk tiring yourself out. Once your tired, you're in trouble. An armored foe simply would have to stand his ground and pivot, defending himself as you try to run around to find an advantage. Also, due to your lack of armor, you risk leaving yourself open while attacking. If you attack him, and he counterattacks -- if both hits land? You are far less likely to damage him, and will very likely also come out of the exchange injured. It would take luck and a lot of skill for a single sword to take down a heavy armored opponent. That this man even had heavy armor would have meant that he had a lot of money, and likely would have had very very good combat training, since he could afford it. Unarmored opponent is at a great disadvantage, generally.
It CAN be done. I'm not saying it's not possible. But you're really going to have to work for it.
Matches between similarly skilled opponents is often determined by who makes the first mistake. In a match where only one opponent is armored and you are not, he can afford slight mistakes, while you cannot. Being forgiven (due to armor) for a slight mistake IS the difference between life and death. It's the reason armor was ever used to begin with.
also from my limited experience i seem to recall that normally you would have either a suit of chain or a suit of mail and any combination would not for the most part be layered except for a few key areas, though correct that if i'm wrong
No, armor was generally layered.
Having done some heavy armored combat, if you are wearing metal of any sort, you want something between the metal and your flesh besides perhaps a tunic or a shirt of some sort. It's metal, if it gets forced into your flesh due to sudden impact, it's going to hurt.
A single piece of chest plate would be less hurtful as you have less flex, though a nice dent jabbing a jagged piece of plate into your flesh will hurt. You want something else under the metal to cushion that. With chain maille (or any number of other variations of armours, such as scale maille or brigandine for example) the chain links, scales, or metal plates getting crushed into your flesh is very very unpleasant and will harm you. I have gotten cuts and bruises from not having proper sleeves on my gambeson when taking a blow on my shoulder - the metal lames (similar to scales) dug into the flesh of my upper arm due to the blow and i was bleeding - not badly, but still cut up from the metal being bashed into my skin.
Certain key areas DID receive additional protection. But that was despite also wearing other layers. I believe I linked them earlier, but I'll link them here as well, and list a relevant quote from each.
Here is an excerpt from the following link:
http://en.wikipedia.org/wiki/Gambeson....is a padded defensive jacket, worn as armour separately, or combined with mail or plate armour.
Here is an excerpt from the following link detailing information that is exactly what this topic is about:
http://en.wikipedia.org/wiki/Plate_armour#Effect_on_weapon_development....the attacker concentrates on these "weak spots", resulting in a fighting style very different from unarmored sword-fighting. Because of this weakness most warriors wore a mail shirt (haubergeon or hauberk) beneath their plate armor (or coat-of-plates). Later, full mail shirts were replaced with mail patches, called goussets, sewn onto a gambeson or arming jacket. Further protection for plate armour was the use of small round plates called besagews that covered the armpit area and couters and poleyns with "wings" to protect the inside of the joint. The evolution of the 14th century plate armor also triggered the development of various polearms. They were designed to deliver a strong impact and concentrate energy on a small area and cause damage through the plate. Maces, war hammers and the hammer-heads of pollaxes (poleaxes) were used to inflict blunt trauma through armour.