Having played both Merc and Commando in PvP fairly regularly I can agree fully with what has been said.
On commando what good is a knockback when melee opponents just leap straight back at you? especially since now favorite of the month rage specs are getting two leaps now with Obliterate. This effectively leaves us with a 4 second single target stun and an interupt. Slows aren't really much help since pretty much anything thats going to let you live long enough is a channelled ability anyway.
My solution would be first of all make the stun grenade a longer distance, so at least after throwing melee's back you can stun one to stop him just launching back at you. There also needs to be one other form of stun/knockback somewhere commando specific but looking at the commando specific powers I am unable to see any other power it could attach to, so possibly decrease the cooldown on concussion charge.
In terms of Merc I haven't played one in a while now since I gave up trying to make it viable so no solutions here.
Or you can get in a group with players with multiple knockbacks, position yourself to where you won't be leaped to easily... These are called strategies, they are something you want to incorporate into your playing style. They should be something that puts you in a better position(pardon the pun) to be successful.