If danger roomed trained only armor skill, shield and dodging, but MUCH much more slowly than it does now, it wouldn't be exploity. I don't see how a training room would make a dwarf a better fighter or more skilled with a weapon though.
It's an exploit because clearly that was never the intention of Upright Spear traps. If it had been, the ridiculous speed with which it trains skills would've been changed. When building a danger room makes training, leaders, teaching and learning skills obsolete, it's pretty obvious it's a cheaty shortcut to super dwarves.
I can see how it would make a dwarf a better fighter, but I probably need to explain what I picture this dwarven danger room to consist of. That Said, I do think they level way to fast.
i picture it as 10 training spears surrounding a dwarf. The spears can rotate around the dwarf, raising, lowering, moving right/left. while moving the spears can poke at the dwarf like a fighter striking at them. Now picture the ends of the spears covered in foam pad that absorbs the impact. Kindof like the stuff stunt training pits are filled with. These would hurt if they hit you naked. Possibly kill if they hit you in the face unarmored (Trust me danger rooms CAN kill). Now put your armor on, metal helm, metal cap, 6 leather hoods and all the assorted other armor and weapons. This armor is heavy which adds additional strain. all the metal and leather over you makes it harder to see and move. You now are alot less likely to die in this training room. This is the setup that I picture for the danger rooms. If you want more danger use real weapons and it would potentially cause real damage.
Now how can this help one train?
well part of this has to do with the way that hand to hand combat works. When doing hand to hand combat with these weapons, it's not just simply swing weapon at enemy. This sounds easy, but these weapons are heavy, have lots of momentum and are not simply something that you can just start a swing from holding the weapon at your side. (i'm obviously talking more about war axes, hammers, and the such) If you were to carry these heavy weapons just out infront of you, it would be fairly easy to take you down. They are so slow and heavy that by the time you get wound up and ready to land your blow, your target can have very easily moved out of the way. to combat this, warriors take these heavy weapons and try to keep them moving constantly so that when they see their opening, they can strike. it allows them to deliver stronger blows and limit their vulunurability by keeping their weapons moving. htey are more vulurable when they are just starting their swings as they weapons are slower. Now picture a dwarf doing this dance keeping his weapon moving and maintaining momentum, while sparring with 1 enemy. That is the sparring practice that dwarfs do during normal training. Now to prepare this dwarf for battlefield combat where he may potentially be surrounded on all sides(that is if things go crappy, we know how often things get dwarfy...), they send these dwarfs to "Danger Rooms" These dwarven recruits fear this. it's like hell week at a frat, or the toughest week of basic, or seals training or what have you. You go to these rooms where they situatate 10 moving spears around you. You need to practice all you know, put everything together and prove your prowness on the battlefield (simulated). The 10 spears are not going to be as smart as 10 guys standing around you actually holding spears. but that isn't the point, it's to get you to look around, get used to fighting back to back, get used to multiple enemies on all sides. Now I can totally see how staying in these rooms till you have your skills honed can help somebody learn how to fight... if a silly militia dwarf didn't read the dwarven survival manual and didn't bring proper face protection then that's just dwarven natural selection.