PVE is really it at end game right now, and they can't tank or DPS, so kinda not worth having around. Not much they can add that the other two can't do, Cryptic missed the boat on making that class useful.
Would you mind clarifying what you mean about by 'PVE is really it at end game'? After nearly finish leveling a Sci character up (at level 42-43), I've found it's pretty damn spiffy. While I haven't completely leveled up a Tac/Escort character (only up to commander), I find that the Sci character generally has a nice balance of agility and resilience.
Have done a couple of STFs with said character, and I find the ability to keep my allies going while shutting down enemy ships invaluable. While the damage output isn't quite as high as my cruiser, I'm still enjoying it.
While I haven't done much leveling with my Tac/Escort character (only up to Commander levels), but generally I don't seem it has the resilience I like in a ship. Maybe when I get to higher levels with it, I'll find the alpha strike capability of the ship will win me over.