deck mud vs concrete for shower base
I'm no mason and have only done basic tile jobs (no shower bases), but here are my suggestions:


Weight: Dry pack is very porous and wouldn't add the 200 lbs. or more to a floor that concrete would. It's also easier to move and carry for the same reason.
Workability: Few have your finishing skills, and dry-pack is more forgiving and easier to shape. A person can plop a level on it to check slope and plane without getting all gooey, too. 
Permanence: Dry pack, being porous, is fairly brittle. It could be trimmed or shaved or completely redone with much less sweat than concrete.