I would tend to agree. As I alluded in my OP, the dread guard-geared-and above 55s doing classic ops are probably after vehicles/pets/crafting materials/achievements
I think the main reason for level 55's doing them (the only reason I do them, at least) is for the Ultimate coms for the Classic Operations weekly. We normally run them in guild, but sometimes I have to pug it.
I also run 50 HM FP's pretty often, as it's an easy source of Basic coms.
I do think that the 50 ops should be in GF, if for no other reason than to let players go do dailies and whatnot instead of sitting on the fleet for a half hour trying to get a group.