I'm not a "super pro" at this root thing, but I do find it useful for certain things.
My pro's of rooting would definitely have to be: 1) Ad blocking (remove ads from webpages/videos). 2) Getting more access/control over my phone [Access file system which is normally off-limits]. 3) Access to more complex apps that do more than the standard non-root apps [Titanium Backup, Lucky Patcher, etc].
The only cons that I've ran into or heard of are: 1) Rooted phones potentially are less secure, especially if you approve an app that is malicious. 2) You can really screw up your phone if you don't read and follow the directions!!! 3) If you flash different ROMs, expect to have to wipe out phone and reinstall apps all the time.
To answer the second part of your question... yes, you will have to revert your phone back to stock if you want Sprint to service your phone. I've heard they won't even look at it if they see that it's rooted.
There are definitely more pro's vs con's for me. I normally just run stock ROM with root capabilities so that I have the extra features if I want. Hope this helps.