PT can put up numbers in PVE, but their utility is terrible. No threat drop whatsoever, no CC, no knockback, the only useful things they have is a pull (which generates threat, which is bad), and a taunt, which has situational utility at best. I'd rather have a marauder, juggernaut, assassin, or operative for a melee DPS. The short cooldown for interrupt is about the only redeeming feature of the PT, and you have to spec into it.
Lack of threat drop is an issue.
The rest is meh though. Pulls/knock backs/cc are not required on any challenging pve content.
Marauders have bloodlust, which is very nice, true, but the rest don't really bring anything substantial other than DPS to an ops fight. Really, ptechs ability to stay at 10m range for almost all of their DPS, and I'd count their ranged attacks as good enough to break even with lack of a charge/sprint. (Rapid shots for a ptech can do 1k+ DPS with combustible proc, plus rail shot.
But as others have said, both ptech and merc are viable in ops.