Further to your point about cheap, traditionally BBMs wouldn't incur text message fees. These days though it seems like carriers offer plans with a sufficient number of text messages for most people... which isn't a surprise given the huge markup that they've always had.
Maybe full keyboards are easier to type on with long nails?
I heard this all the time when I worked at a Verizon. Girls with nails can type at 60+ wpm on the blackberry keyboards, it's unbelievable.
Low cost device paired with BBM, with the major feature being BBM PINs are essentially anonymous and decoupled from the ph#