It’s way too expensive to have clunky micro tied to it. Once the siege tower has properly ungarrisoned over a wall once, until the siege tower moves again, any unit placed into it should automatically ungarrison on the other side.
So, you spend way too much for a unit that doesn’t fight, but in exchange you can swiftly move an army over a wall instead of dropping five at a time. They still suffer from not being able to escape, so I don’t really see why we shouldn’t.
It’s already doable. There isn’t even any need to have unloaded anything yet, just click on a wall and as long as you don’t move them away any unit that gets in the tower is instantly unloaded on the other side.
Cool. Never seen it done, and never used it myself (who can blame me) (answer: anyone who wants to) so I didn’t know. 
Siege Tower is currently unusable cause you send your units in a trap. They are caught inside and if the opponent walls behind they even can’t do any damage to the eco while being trapped.
That’s the main reason, why they are useless.
Garrisoned archers should fire arrows too!
Indeed. That’s why I was considering general QOL changes to benefit them so that they have a little more upside to their dramatic downsides.
One would imagine telling someone they have to give up a second monastery, stable, or siege workshop for one of these on arena (and then some) is deterrent enough.