You need the blackberry-specific data service to use the blackberry branded services. This would include blackberry messenger and the various instant messenger apps published by RIM. The generic mobile data service isn't going to do it for you if you want these features.
You may have some data functionality on the generic data service, but make sure you aren't paying per kB. Talk to your service provider, and see http://www.blackberryfaq.com/index.p...eric_Data_Plan
But you will likely be unhappy with the device without a blackberry data plan.