i'd guess it's somewhat dependant upon your playing style...but having had a DK warrior and an archer hero, i can't imagine having my DK as the 'distance' fighter.
here's why. personally, i wish(ed) the DK wasn't even there. but he is. i like my hero to DO most of the stuff (killing <img src="/ubbthreads/images/graemlins/evilgrin1.gif" alt="" />) and as an archer (set to aggressive mode) my hero was able to kill (i'm guessing) 75-80% of all the monsters. by having the DK as the pack mule (warrior) he seldom even made it into a battle - i got to kill everything myself.
and while i'm not sure how things would be with a magic user, i'd guees it would be about the same. by using the DK as your 'distance' fighter he'd be gaining most of the killing glory, something i'd not like. i play RPGs so that I can play it, not some NPC.
DK was plenty(!) powerful when required. in fact, the game is too dang easy - so don't worry about that part.
just my opinion.