Maybe in tournament play where people actually know what they are doing. In hot join you can still survive fairly well if you just try to avoid taking any damage. During the last tests / betas, I never specced anything at all towards health / armor, and I still didn't die nearly as often as I did when I played as a warrior*. Blurred frenzy and DE combined with distoration already pack a lot of evasion. Indeed, a fly may die if you hit it, but some of them are still damn hard to kill.
I think 2000-2100 toughness should be minimum to make any kind of proper sPvP/PvP build. Anything below that and you die like a fly by anything. However, illusionary membrane goes a long way to protect you in several cases.
I also really think that the best way to counter a thief is offence. The few percentages of damage reduction you may gain from extra toughness aren't going to help here.
*Ok could be because I just suck as a warrior but anyway.
Edited by RandolfRa, 22 August 2012 - 08:23 PM.