Not sure why everyone is so focused on the one skill wonder of merc/commando when other classes aren't really that far from them.
I basically have 3 characters that I play Commando, Shadow, and Sentinel. The first two I listed I have a skill on both of them I spam while keeping 2 other abilities on cooldown (for the most part I mean obviously you have your AoEs, CCs, and defensive cd's). I know first hand also that sages/sorcs have a spamable ability though I don't know how effective it is for damage or if they are just really bad for doing it.
Sentinel though is a bit different since there are a few more abilities to keep on CD, but after everything is on CD and you have focus there is only one thing left to spam which you end up doing.
Every class but the merc/commando have an interupt so if you aren't using yours when facing said classes on their tracer/gravs thats your fault.