I have never used BBM, so I guess that I don't really get it. Everyone I know has unlimited sms / mms, why would I want to use an app for messaging and eat away at my data plan (although an insignificant amount of data I would imagine)?
My buddy who works for RIM answers my question with "speed and reliability". However, I don't have reliability issues with text messaging, and via testing with friends, SMS appears to be pretty much instantaneous.
Now Laura's comment about international messaging is a good point, I've never thought of that.
I hate to see RIM / Blackberry fail. The BB10 OS is fantastic on paper, the technical merits can be praised, but in typical real world use, it really doesn't seem any better than the others on the market. I'd love to support Blackberry by owning a BB10 handset, but WP8 genuinely won me over fair and square. I love the UI of WP8, and I just don't like the aesthetics of BB10 hardware design. While I appreciate the functionality that BB10 UI offers, I don't like the UI design either (mainly the icons).