The two reasons I did it were to remove Sprint's bloat and install wireless tether and not have to pay an additional fee for the data I already purchase.
I didn't think a factory reset lost root either, that is if you permanently rooted and not just a temp root by using Vision or z4root. I assumed you could just redownload superuser and you would be good to go. I thought you had to run the RUU to get unrooted from a permanent root. Maybe it does unroot you but you still have an unlocked bootloader so you wouldn't have to go through all the steps to perm root again?
Factory reset does not un root your Shift you have to run the Sprint RUU to un root.
I Rooted my EVO Shift for a lot of the reasons listed the cons listed are true as well. I think by rooting I have unlocked the true potential of my device and I have the freedom now to do what I want with it things like Over/Under clocking, theming, running custom ROMs and all the great apps like SetCPU that lets you slow down your CPU and save battery. There are benefits and drawbacks ether way. Bottom line... if you don't like it you can always put it back the way it was.