The thing is, people run around in a Darkness spec in DPS gear, play as if they were a DPS but just throw a guard on someone, then complain they dont feel like a tank.
Assassin tanks in tank gear vs tank juggs.... The assassin is easier to kill straight up, yes. But he has other things to make up for it. Stealth. More CC's (especially if you spec into intant whirlwind). Force speed. Overload and Force Pull instead of Force Push and Jump. And of course, being able to do more damage than a jugg tank. The difference is not "little" as you say but is VERY big - Im talking being able to easily do 2 or 3 times as much damage in full tank gear compared to a full tank jugg, while being able to put up similar protection or more.
What you have to understand is that playing as tank in full tank gear is a COMPLETELY different playstyle than playing in tank spec with DPS gear as more of a hybrid DPS. Few people actually do the full tank sin, and even fewer do it well.