It's understandable when there is five enemies surrounding you and focusing all there effort on one particular player, however that isn't always the case, there have been times when I'm squaring off against a player with full DPS capabilities and he manages to take me down with ease, some of there abilities seem to take a major toll on my HP than others, lets take a bounty hunter's tracer missiles for example or a operatives backstab, those right there deplete my health bar rather rapidly.
That is the reason most SIN tanks use DPS gear.
Your defensive rating and shield really only help you against marauders / juggs, and sins backstabs / rail shot etc. White damage.
Tech attacks like tracer missile, blade storm / force scream, smash, flame burst will still take their toll on you.
And you also get a move that negates them for 5 seconds.
Really shadows survivability, if you add in the ability to cloak out and heal up, is far better than any other tank.