Hopefully, all plugins and themes should function well. I don’t know how important really was the upgrade, but I’d rather comply.
About the upgrade:
WordPress announced an update to its blogging software from the current 2.3.2 version (released about a month ago) to 2.3.3 (release about 30 hours ago). It is supposed to correct some security flaws and fixed some minor bugs (like being able to edit other user’s posts on the same blog, I supposed). For more information, check the official release info site.
Consequently, all MU installations also have to be upgraded to 1.3.3. Click here for more information.