I find it horrible that most people actually try to do this and 'gain' and advantage in traffic. They are merely saving seconds. I usually refuse to allow them back in the line if they try to pull that. Simply tail-gate the person in front of you, I've had it.
Totally being a devil's advocate here, but here goes.
How do you know if they are "back in the line" or actually merging? Especially if the merge lane is 1km long or more and it happens at the end? There is no way you can tell and by tail-gating you are 1) violating road rules 2) just being a d1ck if the car is actually trying to merge
What do you do if the person in front of you, in the right lane, is yapping on the phone or fixing her (or his) makeup or fiddling with their papers in the passenger seat leaving a 5-car length gap in front? You are crawling behind watching as one, two, three, four or more cars are merging in front of this d1ckhead. Sooner or later you will either honk (usually no effect) or simply "jump" in the merge lane to overtake this slowmo on the right. Not quite legal but the "legal" option will drive you nuts.
Having said that, these jumpers drive me crazy too. Bayshore to Woodroffe is the worst and I seriously wish there were OPP there at all times - they are an effective deterrent.