yes, there are plenty reasons to do a JB... at the same token, if you're happy with your iDevice as-is don't do it.
i always JB my 3G... then i got to testing beta's of iOS4, which were no JB for (except beta 4 which i did JB) then i switched to a 3G, didn't have it long enough to JB (5 wks)... now i have the iPhone 4 and i'm debating w/ myself to do it or not. i blog about Apple product incl. the iPhone and just to have done it so i can discuss it might be a reason for me to do it....
the things i used to JB for; multitasking (now in the OS) wallpaper on Springboard (now in OS) theme Springboard - only think i don't like currently on the SB is the labels, they limit the wallpapers that can be used in my opinion. biteSMS, much better imo as the default Messages.app. Lockinfo, i don't like having to unlock my phone and go in the Mail.app to see who emailed me (using Prowl/Growl now, does the trick but only if MacBook is on) so yea, i have some reasons to JB; but i don't want to make a very stable snappy fast iPhone into a buggier, not quite as snappy iPhone... we'll see which side wins :}