The archer will:(1) give you access to Disarm Trap,
(2) give your party a bit of muscle in the dangerous early stages of the game where the rest of the party is still a bit wimpish,
(3) be able to wear some of the gorgeous goodies banned to the magic users, and
(4) be able to use a bow properly - a very important weapon in all the Might & Magic games.
OK, the archer doesn't get access to the higher-level Dark Magic spells, but I think the character's other advantages more than compensate in this game. Stålhand and I have been having an on-and-off "debate" over the pros and cons of archers for a few years ... *evil grin*