From the FAQ. Here is why you need a BB dataplan
What do I need a Data Plan for? What do I need a Data Plan for? - BlackBerryFAQ
* Do I need a data plan to get email?
Yes, without a data plan all you have is phone, SMS, and PIM services. (ie... Outlook, Contacts, Calendar ect..)
* Do I need a data plan to access the internet?
Yes you will need a data plan to access the internet
* Do I need a data plan for MMS?
Yes, on most carriers, you will need a data plan to send/retrieve MMS messages.
With NO data plan you may:
* make phone calls
* send/receive text SMS messages
* NOT use any instant messenger program that anyone has found so far
* use the device as a standalone PIM
* NOT send or receive any information over the internet, in any way shape or form, despite what you heard about it being possible
With a generic data plan in addition you may:
* use a third party browser like Opera Mini, but NOT the resident BlackBerry browser
* may get email either via a web interface (using Opera) or other third party app, but NOT BlackBerry push email
* use a number of third party applications that require net access, including some instant messenger programs, as long as they're available to install from DM because:
* canNOT install apps OTA because that requires the resident browser
* canNOT use BlackBerry messenger or do PIN-->PIN messaging
With a BlackBerry data plan you may do all of the above plus:
* receive true BlackBerry push email from up to 10 different email accounts
* use the resident BlackBerry supplied browser to access the internet
* install applications OTA via the supplied resident browser
* use BlackBerry Messenger and send PIN-->PIN messages