A.- Shadows/Assassins aren't the only tanks and B.- it's not a matter of "needing" DPS gear, it's a matter of optimizing stats in situations where extra endurance will do you no good.
As a Shadow tank, I have yet to find a situation where extra endurance/health does me no good. Since I'm not in the business of losing aggro, it helps to provide the healer with a bit more leeway.
On the guard topic: I always have it on the DPS except in the situations, operations boss fights, where it's useless to guard DPS so it gets cast on the healer. I always ask for guard when I'm on my sentinel. Bad pug healers will whine, bad pug tanks will guard the healer. When I do get guard, however, I go ******* crazy and try to steal aggro off the tank just for the fun of it. I do announce that beforehand and people respond nicely so we enter a contest of sorts. They do their best to keep aggro, I do my best do steal it. Keeps the flashpoint a bit more interesting.