I don’t know… In theory, he’s got a lot of potential, but it’s just hard for me to find a place for him in the party and make him work — while other ‘classic’ Holy mercs work so well together and complement each other, with this one, it feels awkward — you might as well be carrying Illidan with the team instead.
True, although, ironically, the rest of the team is more about AoE, healing and protection, so he sticks out a bit…
The ‘classic’ team is built around the combo of Anduin and Velen, which is still good and which, ironically, Tyrael doesn’t synergise with that much. Together with Xyrella, they can still do a lot — think of heroic bosses such as Coren Direbrew, Omokk, Lokholar, Echo of Medivh and many others; in other situations, good old Cornelius is excellent instead, getting many jobs done. And can the same be said about newer mercs, especially Tyrael (Yrel can at least support the duo with more stats, spell and healing power, if nothing else; against heroic Onyxia this is, perhaps, the easiest method)? Can they at least perform the same tasks that the old team could or, perhaps, accomplish something else that couldn’t be done before? And lastly: who do you play Tyrael with? Yrel, probably, and Xyrella or Cornelius? This feels a bit awkward, and it’s unclear against which opponents this is optimal.
Try Balinda, as in my old version of the team. No cooldown on that lesser water elemental, no questions asked, a decent AoE available on turn 2, which synergises with the double nova, and quick darts to finish off some potential targets. I think she’s still better than even Yrel in that team sometimes, and she can do heroic Onyxia with them, too (I would open there with Velen, Cornelius in the middle and Anduin, take out the red warders first, and, if Cornelius goes down, replace him with a green merc, putting Anduin in the middle).
PS An afterthought: take Scabbs Cutterbutter, give him an SI:7 contract — that’s something like your Tyrael in terms of ‘burst damage’ to a target, except arguably better in some situations. 
PPS Had a task from Tyrael to complete the Dragonbone Golem bounty (normal, with Hooktusk in the party), so I gave him a go — besides, Holy is how I would normally do it anyway (Velen, Cornelius and Anduin for the boss fight, took also Xyrella for climbing in case I needed her). On that difficuty, Tyrael was destoying blue ‘trash mobs’ like those Lightmaw Netherdrakes or Dragonkin Sorcerers in one hit — not bad.
Oh, by the way: not long ago I would look for Tyrael coins or even resist the temptation to accept a trade offer, and now they are just… Renown. So, once again, my advice would be to not rush it — eventually, if you play the game, your mercenaries tend to get there, to that ‘maxed’ state.