Hmm, but that's if I'm fighting from behind it (with melee?). I'm planning not on fighting, but healing. I'd get the total cover bonus then right? And 9/10 cover for firing my crossbow from it?
If you are healing someone in front of you, you would still only get 1/2 cover. However, if you moved into the same "square" as a fallen comrade, you could use the shield to give you both full cover from the front while you healed him/her. Keep in mind that unlike a normal shield, which functions in all directions, over the course of a round, tower shields only operate as cover in a given direction.
You could use 3/4 cover and still shoot a crossbow. However, reloading while holding a tower shield isn't possible.
A tower shield's best use is during siege warfare, or in open battles, where a line of soldiers can lock shields in front of a line of bowmen, for example.
They really aren't extremely useful to the wandering adventurer, except in certain very specific situations.
Cheers,