As an occasional magplar, Extended Ritual is a skill I don't get. It seems to be regarded as essential, with PvPers scarcely giving that a second thought. Kristofer ESO's Ghost build could be played on any class, but I think he misses the purge terribly when he's not on stamplar. People like Isth3reno1else, also on YouTube, if they're going to single out a templar skill as OP and overloaded, it's this one. Well, I don't get it and these are my counterarguments:
(1) The skill is terribly expensive. It lasts a long time as a heal, but that only comes into play, if you are staying put in one place. Well, I'm constantly on the move, so what good is that? People say it's huge but, errr, no it's not. The stay in your house idea was never a good one and has been thoroughly superseded by sources of speed in the current game (Steed, Swift, RAT) and by Toppling Charge having become a good and popular skill. Magplar plays like quite the mobile stamina class these days. I much prefer Radiating Regeneration. It's far cheaper and stays with you. In fact, with a vMA resto it's basically free.
(2) As a snare removal tool I find it completely useless, because it provides no immunity and, again, is far too expensive. It's no substitute for RAT or Mist. Furthermore, I am now playing my templar with Sugar Skulls, 16K stamina and Bloodspawn, but it's no good. Once you run out of stamina, you're out, and it doesn't come back fast enough for my playstyle, which favors some dodge rolls. This may have something to do with me not playing vampires nor using Mist. At any rate, because of this, I find Ritual competes with Meditate for bar space. I highly value stam regen, more than the purge basically. I have 1.3K on my magblade. Whichever way you slice it, in order to get such stam sustain, you're going to make sacrifices in your gear, in your magicka sustain by switching to Restoring Focus, or you have to slot Meditate. Basically for my playstyle this beats Extended Ritual and you compromise your damage or your skill bars too much by incorporating both.
(3) I find purges very situational. Yes, it's a good move in some situations, but casting Ritual costs a GCD and is more expensive than Honor the Dead. It is not something I've ever found all that useful when I can heal through the damage like every other class. In fact templar's problem used to be a lack of healing over time and this may have been the reason ZOS added such a strong HOT to Ritual. This is what allows you to stay on attack and what makes it less likely you're backed into heal spam, but if you're strictly after the HOT, Regeneration is better. Fine, if you want a 1H+S back bar, then I would go for Ritual, but there are other advantages to a resto staff. It plays well to activate a Berserker enchant, for example, and it's better for restoring magicka than a lightning staff.
(4) The snare is not something I even think about. I suppose that's because my main is a RAT-spamming magblade. Against that build it's a complete non-issue. I don't think it adds much to Ritual, but I could be wrong.
I play solo or in small PuGs in CP Cyro and IC. Paradoxically I use Extended Ritual on stamplar as my magicka dump, but on magplar ... meh. I'm not saying I'm correct. Even in the context of my build and playstyle, I'm not sure what I'd end up with if I played templar for longer stints than I do. Let me know why you're running Extended Ritual and how your playstyle possibly differs from mine. Is it more about group play, stacking rituals and the synergy for example?
PC EU (EP): Magicka NB (main), Stamina DK, Magicka Warden, Magicka and Stamina Templar, Undecided Sorcerer (currently hybrid)
PC EU (AD): Woodelf NB, currently magicka
PC NA (EP): Magicka NB