BIS helps to "push" your emails to your device (yes up to 10 email accounts). That way you receive the emails as they come in -- you don't have to actively check your inbox to see if anything new has arrived.
I'm not too sure how Sprint does it, but most mobile companies have a standard internet/data/web plan for BlackBerry which is basically the BIS plan. For non-BB devices, they also have another data plan -- when you say you have a data plan are you referring to BIS or a normal non-BB data plan?
If you have BIS, then BIS is necessary in order to use Gmail Mobile only because BIS is like your ISP.