Sure there is. I have a perfectly stable JB on 4.0, and 4.2 is what, maybe a month away? If you enjoy the whole process, great, but not everyone wants to go through the whole restore/upgrade/JB and possibly unlock routine, having to redo everything, for every single minuscule update Apple wants us to. Especially when the next round is in sight already.
Hell, I didn't upgrade my phones every time even before I JB'ed. LOL
Personally, I don't find 4.1's "upgrades" compelling enough to spend my time going through the whole routine again, especially since it's another step to protect the baseband this time around, and I don't have prox sensor issues to fix. Add those upgrades to the ones from 4.2, and maybe there's enough there to make it more appealing. Yeah, it'll only take a few minutes if everything goes right, but when it comes to computers, I never assume it's always going to go right.
That said, if the feature you want (or a fix you need) is only available in 4.1, you might as well update now that there's so many JB tools available, and now that PwnageTool is available to keep you from updating the baseband.