Honestly, if I still had my 2, I wouldn't. No Passbook, no Siri, no YouTube app, no Google maps app? Not enough reasons to update yet so many reasons not too. Unless my 2 was crashing all the time on 5.1, I don't see a reason to update.
My personal pref on updating is not to update a non current issue device as the new OS is usually optimized to get the most out of the latest & greatest, yet makes older devices more buggy. At least that has been my experience with iPods & iPhones. I don't see the iPad as being any different.