Depends on what you want...
Reasons to Root :
Overclocking (My Droid is at 1 Ghz, stock is 550 Mghz). Please note, certain phones overclock better than others due to processor limitations. I can get as high as 1.25 on my phone, but it gets a bit hot.
Wireless Tether : Can do with a rooted phone
Run OSes before they release for your phone (I am currently on Android 2.2, which has not yet been released for my phone. @ Lee, I believe you have an Evo, if so, there are 2.2 roms for your phone as well)
Completely mod your phone should you have the time (for example, I pulled the "launcher" apk from my phone, replaced the launcher icon with the "LA Lakers" logo...)
Here are some other reasons, and on top of these, there are a ton more reasons. Really, IMO, if you like tweaking your PC, you will like tweaking your phone (but that is just IMO)...
http://calebnance.com/contact-us/blo...-taypotts.html
I have added a warning to the OP, but really IMO it isn't necessary...It is VERY difficult to completely brick your phone by rooting..Almost impossible...