Honestly, classic is a boxers dream from a PvE perspective. It's the only version of the game where it's really feasible to box at all levels of content. Many high level heroics/mythics in modern expansions are just too complex and have too many mechanics to consider boxing.

Even a moderately skilled player should be able to manage 5 characters in a classic raid environment since the most you ever have to monitor as DPS is not standing in fire and/or decursing occasionally.