Towers die permanently. People don't.
You can heal players rather easily. Not so with towers.
You can heal towers. 2 ways.
1 get tower health and regen from citadel upgrade.
OR, my favourite and I have saved many a tower ingame - But the Universal gadget from the item shop. Will heal allied Demigod OR Structure (TOWERS!) fro 2000hp.
Also, with a reg player on your team, lay some mines at UBs spit range near the tower to make him think twice about it.
The idea of towers dying permanently is it gives another element to the strategy of the game... Do you save towers, capture flags, destroy towers, farm minions, kill demis, upgrade citadel, buy gear or items?... these are all strategic based decisions ingame that make this game so excellent.