Jb opens up security holes, as the Cydia apps are no longer checked by Apple, this is especially true for cracked apps. But I guess this is the case for all operating systems.
The only reason I use jb is, because the iPad is currently my only internet connection; so I need MyWi for tethering. But I will go back to the official firmware as soon as either the BT engineer gets his lazy ass over here and sets up broadband, or Apple releases 4.3 with personal hotspot functionality. Bottom-line is, I feel safer without jb.