Elders can dismiss troops in the bank? And still with the 'mark every troop in the bank with their donator'... Talk about unnecessary extra work, even if it would not cause too much server burden, who would rather their server do more work when another system is working perfectly fine?
Elders or higher can just dismiss troops donated in the clan bank might be able to counter the problem with useless troops flooding the bank, but what would happen when an elder (or higher) went into trolling mode? Weeks worth of hard earned troops would disappear into thin air. Sure you could say that only responsible people should be made into elder+ in a clan, but that is just not possible, some trolls are bound to get the promotions.
Another possible issue is that this enables self-donation. I put in 2 witches, a wizard and 2 archers in the troop bank and withdraw them myself. No need for clanmates. You could say that troops donated by a player cannot be withdrawn by the same player, I go back to my 'why more unnecessary work' arguement.
Another game actually uses this "troop bank" system, exactly like you mentioned here, but completely disregard the levels of donated troops and withdrawn troops follows the withdrawing player's troop level. (Lvl 1 if the player does not have the selected troop). I have been playing this game I mentioned for almost 2 months now.
IMO, troop withdrawn is made to equal the level of what the player have is the most realistic and simplest solution for a "troop bank" system. No unnecassary saving and marking of who donated what thingy. But I gotta say, I prefer if THAT game uses the current coc donating system than a banking system, you could get higher leveled troops than what you have.
From my experience playing the "game that uses troop bank system", I know a lot about the restrictions they placed in their banks for donating and withdrawing, but the whole guild system there is very very different to clans in coc.

