I play solo a lot and I don't use the jobs much. In a group setting it does change and jobs become more important. This is only 40+ and becomes more pronounced with higher gear. ( many jobs get class defining abilities at lvl 40 and there is no similar skill available from cross classing)

A difference of 10% to 15% dps/heal/threat is not unreasonable at ilvl90. While the original question focused on summoner, all classes face near the same issue. Unless the group is over geared, you are asking others and yourself to work harder in a game.