Is the templar useful in the three dlc areas?
In regards to Descent and Trespasser (both having hordes of enemies), I'd say Templar is more useful than Reaver or Champion because Wrath of Heaven is an AoE stun ability. Compared to reaver, who has a random proc fear, or champion who has...nothing.
In regards to Jaws of Hakkon, the only tank who did not suffer sever zombie-brainification from repeat resurrections, was Cassandra. Her AoE stun, along with other standard warrior abilities (aoe fear, aoe party-wide guard buff, knock-downs), and spell purge made her the pretty unkillable. Especially given the new toggle on Wrath of Heaven than heals her. Spell purge is a godsend for removing Hakkon debuffs (which will one-shot anyone).
Descent and Trespasser both have a lot of enemies. The crowd control and self-healing of the templar are just godly.
Pro-tip: don't use blessed blades. It's a total waste IMHO.