the phone works with just a data plan. I dont see a reason why you wouldnt get the blackberry plan since its the same price as the regular data plan (BIS not BES) without a BB plan you are not able to use Blackberry Messenger though.
you don't have to have one to use the phone. But most of the features that make a blackberry what it is involves having a data plan. But the phone works perfectly fine without it.