Even without optimization combat as a melee character in 3.5 is dreadfully dull compared to spellcasters/martial characters/4E. I mean, sure, you have some shitty combat things like grapple or trip, that will more likely then not fuck you over for trying to do them if you're not built to do them, and then be pretty much pointless in anything but at low levels. Or you can hit something with your sword. And maybe you'll kill it about 20 turns or so, as your sorcerer buddy is machine gunning out spells that destroy what you're fighting in a single action. You don't need to be a 'muchkin' to realize that's a shitty situation to be in. Hell, frankly I find it hurts the roleplaying a lot more then optimization does, when you you're suppose to be the wizards companion, but realistically you're more akin to a pet.
I mean, it's not a omnipresent issue. You can play a optimized character, but that's boring as a melee character generally, because that means just doing one thing quite well, but not having any other realistic options (and you're totally fucked if you need to fight something immune to your one thing) Or can play a martial class, which are powerful enough to keep up quite well to spell casters (assuming the spell casters are not optimizing) AND you have plenty of fun options that actually do different things. Of course, martial characters are basically 4E characters.
Edit: Or the whole party can play on a lower level of power, which is often makes quite a good game, but many people don't like doing that for some reason. 5E is a bit better in that regard... Although not... Well, it doesn't feel like that issue is as solved in it as it is in 4E, but at least with the squished power levels you're less a mascot and more a contributing member of the party. Although it seems like you can still be pretty one dimensional, especially if you're one of the free classes (seriously, the Champion archetype is such a joke, and really made me despair for 5E until I downloaded the actual book and read the other options.)