Why is DPS the only measure here? Damage done is, after all, the final measure of how a boss dies. If a casting class has to constantly move to avoid damage and ends up with a higher DPS but lower damage done than a melee class who is able to maintain a constant rate of attack (hypothetical) ...... the melee was at least as valuable.

Stated another way, if the highest DPS isn't the highest damage done then there was a throughput issue with the high DPS toon (ran out of mana, had to move more than other classes, whatever). Now, if you are lowest in DPS and damage done.... you were the weak link.