Yeah, the speed advantage on Marathon only really works for aggressive games, where Marathon greatly speeds up troop movement rates relative to turn speed. Now that anarchy time has been patched, epic speed seems better to me than Marathon for builder-type games.
If there is some kind of limit on game speeds--I can see both sides to the argument--I would hope that we aren't forced into quite those narrow of restrictions. Why not at least allow normal speed for tiny and duel maps, for example?