The thing about being able to harm an enemy crew is as stated above, "Like asking for boarding." It just would mesh well with the current gameplay, nor would it be very enjoyable. While sure someone might find it enjoyable but it would mostly be a nuisance for players, because:
Injuring crew is more a matter of realism rather then gameplay, sure it could it work but that is an entirely new block of code they would need to write. Not only that but this isn't played on a crew vs crew basis, this is about airships squaring off against one another in a manner that best suits the game, IE: Airships vs Airships, it seems like something that would be fun only for a little while but would then become a hindrance in actual gameplay, because then you're winning, not by skill or strategy but by making it so the other guy can not fight back for X amount of time because he needs to respawn.
As for breaking ships apart piece by piece:
The matter of ships breaking apart has already been discussed in a less serious manner. If pieces are going to be falling everywhere they would need to add enhanced physics, it would also break the game in more ways then one. Firstly if all your guns were knocked off the ship you would be able to do nothing, effectively disabled unless you died, secondly it could block off engineering items, having no way to repair something is not fun, being completely disabled and able to do nothing is not fun, sure that last one is doable in the state the game is now but at least then you're able to do something about it. Thirdly, if these things where not physics based how would you do it, scripted pieces falling off certain areas? That's no fun either, because then they would be memorized by players "Oh this part is going to be destroyed at X amount of health. Gotta remember that." If they are physics based they'd have to make the models destructible, that in its self is a lot more work then it's worth in my opinion.