Yeah, getting them into position fast is excellent - as it is with offensive units.
The best part is being able to shift a line without defensive loss (outside terrain). What I mean is, unfortify them, move them up a tile, fortify them - in one turn (terrain permitting). Having to move a few at a time and waiting to fortify can amount to heavy losses or a non-blitzkreig attack. Still, I rather have an offensive unique unit then a 2 move defensive one, specifically like the impi.