I've heard Powertechs can put up numbers as good as any other class, but I really don't see any reason you would want a powertech over a marauder, juggernaut, assassin, or even an operative (rare as they are) for melee dps.
PT/VG DPS deals the same amount of damage as a Marauder and more than an Assassin or Juggernaut and *way* more than an Operative. In addition, they've also got *way* better AoE DPS than any of the aforementioned classes (DfA, Flamethrower, Explosive Dart). Since general utility in the form of CC/stuns/etc. doesn't really matter in Ops all that much, not having them doesn't really make all that much difference. The lack of a threat drop doesn't mean much since, unless your tank is simply bad, after the first 30 seconds, threat becomes a non-issue. Not having a gap closer is only of moderate consequence when you have some decent functionality up to 30m range, especially when you're DoT based. It's annoying, but nothing that is going to make them incapable of fulfilling the given role.
In short, PT/VG DPS is raid viable because it puts out *amazing* damage numbers and not much else matters.