If I want a read/write disk image, then aren't sparse bundle disk images always the way to go? They start at 0 kb in size, and grow to accommodate whatever you add to them, and if you delete stuff, they don't shrink automatically, but you can run a command to shrink them manually to regain space.
So what advantages does a writable .dmg have against a writable .sparsebundle or .sparseimage?