RIM already provides IM clients for Google Talk, Yahoo, and MSN, with pretty much the exact same UI and features as BBM. The quality of their Google Talk client is one of the things keeping me on BlackBerry.
Blackberry Messenger (BBM) also has a typing indicator, delivered and read indicators, and it's faster than anything else out there.
Also, the fact that it's only on mobile devices and not desktop computers means that you know your message will get delivered to someone's pocket instead of the computer they may have left on. An odd side effect of its lack of portability.
I had been thinking about creating BBM for other platforms simply because I'd like to have it on my iPhone.
Their reliability isn't great, however.
GTalk/MSN etc = Mobile - Desktop Messaging
The use cases are completely different.