My two cents:
for me, the main reason to root my phone (apart from it being a first step towards using custom ROMs, as already mentioned) is the ability to use DroidWall, which allows me to restrict network access for particular applications.
I'm coming from Hero, same as Vera above, and I agree that being able to get rid of the big, slow and ugly Sense was a godsend. Battery life up, interface responsiveness through the roof, and with Android 2.3 on the Hero, the whole thing got even faster.
I recently bought my IncreS, and I expect it to follow similar path in the future, when its hardware starts losing breath compared to newer and newer phones and OS versions.
The more you depend on forces outside yourself, the more you are dominated by them.