I don't mean deacking with the rockets, I mean you can MG the acks without dying as quick as the M3. Maybe I'm wrong, I haven't done it too many times. The M3 doesn't seem as survivable versus acks. If you can be in the ack where the M3 can't, that's some useful advantage. Only works if the defenders are busy returning fire, though.
If you're sneaking, the ack needs to be taken down. Other than an M8, the 251 should be able to keep up with anything that's doing the deacking.
Those rockets are such a large HE package.. It's a shame to see them unused.