The main draw of the BIS plan is its push email feature. It seems that when Sprint used that wording "In order to receive emails..." they are using receiving in the "push" sense. So without a BIS plan, you can't really have email pushed to your device.
FYI: customers who buy a BlackBerry through Verizon HAVE to get a BIS plan. Tmobile on the other hand is a lot more flexible. You dont have to get one if you dont want to.