it does drain battery life more than without it but i am honestly surprised it made that big a difference maybe theres something in the settings of the group to let you download everything every 5 minutes instead of real time, I know there is with emails makes sense there would be an option for group bbm also.
I thought that but after spending hours looking and changing settings I had to seize control back by killing BBM off. It would be simpler if you could actually exit BBM and start it up when you want a browse through the posts.