it's not only about starting gear. it's about gems, enchants, hc gear and raid gear.
it might take a while do complete quests on blackrock but they earn marks and honor while doing so, which isn't the case for onyxia players who do hc or raids. also, many players on onyxia who are mainly interested in pvp don't raid on a regular basis for several reasons. they might not be in raiding guilds, have time zones where it's hard to find raid groups, etc. pretending clearing kara is a 3h time investment for the average ony player is just not correct.
if there would only be the issue of starting gear, this would be resolved pretty fast and onyxia and blackrock players would be on the same level (gearwise). buut this is not the case as anyone who does bgs and arenas must have noticed. having a full ony group vs a full blackrock group in bgs is an auto loss unless the ony group is a premade.
if people on blackrock are oh so competitive, they should be against the merger aswell because farming low geared people shouldn't be satisfying for a truly competitive minded player.