I use it once in a rare while if I'm just need a quick adjustment to something. Faster than loading up zb and sending over, then sending back and all that. 

But mostly I use zb for any real morphing work.

But a lot of folks here don't have zb, or don't use another sculpting program, so in that case, the morph brush comes in handy.

Also, a pen tablet is useful for painting weight maps.