spawn arme a quite a bit bigger than marine arms. hell, some of the spawn arms are bigger than a whole marine
why you don't like spawn DK? being able to move and charge potentially 24" in a single turn is not something to pass up easily. they may not do too well against MEq armies but they'll cut up ig eldar and tau rather well
oh, and on the point of daemons, they may not be able to move the turn they deep strike but they can launch an assult. handy thing to remember that