Jail cells are serving multiple purposes. For one, it provides you with alternative routes instead of funneling all players into one entryway. This benefits the attackers, don't you think? Second, when tower ownership is changed the gates respawn and provide defenders with a buffer of time to organize themselves while the gates need to be destroyed.

Is your only concern the speed of getting back into the fray? It was designed specifically to get attackers back into towering as quick as possible, contrasting to the pretty poor design of Destiny's respawn mechanics. I can't imagine it being much faster if you were spawning outside of the tower and being bottlenecked through a single small entrance.

Aside from towering it helps keep the random no-pk areas on the overworld limited.