The battery pull will essentially reboot your phone if you need that for any reason. Do it while the phone is on. On occasion I will view messages/emails and the phone will still show x unread messages. A battery pull will usually fix that. The other reason to do it is to increase "free memory" on the phone (to see this: Options>Status shows memory free, move decimal 6 places to the left for MB). However the 8900 has twice the memory of previous BBs (128mb) so I don't think you need to worry about this myself. In fact my wife refuses to do a battery pull on her Blackberries and they work just fine. So I say if something seems screwy with the phone do a battery pull, otherwise don't.
BB 8900 Tmobile, Moto Droid Verizon