Templar Tanks do have the benefit of Restoring Focus which returns Stamina every second even while blocking, the problem is that sustain is a big issue even with this really nice sustain skill because of the extremely expensive and weak abilities meaning you spend more resources for less effective benefits compared to every single other Tank class.
There are a few group buffs from the Templar Tank, you have Minor Sorcery which adds about 2k more DPS. Templar Tanks also have Radiant Aura which is an Area of Effect Minor Magickasteal which is great for add pulls meaning Magicka Damage Dealers can sustain better when dealing with add pulls.
You have Purify which is a synergy that becomes available to allies stood inside the Extended Ritual that helps them cleanse negative effects. Extended Ritual is also really good and makes Templar Tanks better than other classes at utilising gear sets like Symphony of Blades, Spell Power Cure, Earthgore and other sets that require group healing.
Templar Tanks are not hugely common due to having so many obstacles and also, most groups tend to use Templar Healers negating the need for a Templar Tank. In the rare occasion you are in a Magicka group, with no other Templar then it would be useful to have a Templar Tank, most likely as an Off Tank to provide the Minor Sorcery.