I don't know about any one else, but jailbreaking was a requirement for a fully-functional iPhone, at least for me. I used jailbreaking for: multitasking ala Palm Pre using multifl0w, scripting in Ruby and Python incl GUI, ability for me to setup an SSH tunnel so that I can get to my phone even when the phone is only EDGE/3G, being able to transfer files to/from my phone via SSH, ability to transfer music to my phone without using iTunes, manipulation of my SMS database, theming, and increased usability with tools like SBSettings, GRiP and many others.
