Dread Shadow is still a great tool in many raid fights, though on T3 you don't really need it that much imo. It's useful on Toth Amon and Louhi, and eventually nothing else imo.
Gyas and Arbanus are "broken", they take way more magic damage than normal (because they probably have no protection, like trash mobs), and thus Tali Tanking works really fine.
Especially Gyas can be solo tanked by a Tali DT quite easily. Arbanus is harder to solo tank (need good raid DPS and good heals), but if you have 2-3 tali DT to switch aggro works perfect.
Hator-Ka and 3 brothers can be tanked with Tali spec aswell (Wyrm tali is really nice to have for magic bosses).
In T4 imo DS is nice to have on General, Emperor and Strom.
I never tanked projections, but probably on those aswell.
But Basilisk, Imp, Zodiac, Emperor Golems (these suffer shitloads of magic damage.. try) and rest of Memory Cloud bosses tali tanking works fine.
For some stuff, if you are scared of full Desacration build try something like this: http://goo.gl/gLB5pj