BBMessenger is one of the few features remaining locking non-business BB users into the platform. They've successfully added a new feature in BB Messenger Music (BBMM?) that works in concert with it to bring users who might use a Bleakberry but not BBM into the fold. Now they'll be getting pressure from any friends who use BBMM to hop on it and share their music so that those friends can have 25 more songs for free and it might seem reasonable because they'll get free access to 25 songs per friend, in turn. Social pressure can be a powerful thing. If your friend leaves who has a song you like but haven't purchased, yet, you must buy that song or find another friend to regain access to it(?). So, when you're in a phone-buying situation, you'll have that additional consideration holding you back from switching platforms (on top of the unfamiliarity of the other platform, potentially outrageous ETFs or unsubsidized purchase prices, contacts transfer, and all the other things going through your mind). Uncertainty motivates people to maintain the status quo.
I remember watching a poor Circuit City salesperson one day deal with a lady and her bratty daughter who kept whining about whether the new computer Mom was buying her would have AIM. She didn't care what else it did, how much it cost, and didn't bother to look into the fact that she didn't even have to be a paying AOL member to have access to it from *any* computer. The salesperson could have pointed at the most outrageously-expensive model in the store and said, "I know for a fact that one will run AIM," and Mom probably would have bought it just to get her daughter to shut it. Silly perceived lock-in situations like that keep companies (AOL, Circuit City) in business long after they've stopped competing when they occur as part of the buying decision.
On the plus side, BBMM has the potential to improve music discovery, which IMHO stinks with existing on-line music stores, assuming wide adoption (Could it possibly be worse than Ping?). More pragmatically, however, if RIM can have one more silly thing locking people into their platform, they want it because they're hemmorhaging market share. Whether its actually good for consumers - as partnering with an open platform like gchat might be - is secondary to their business interest as they try to remain afloat in a sea of innovative competitors.