I do think people are doing it for mechanical reasons. I do think it is very hard to justify. I don't think selling your soul for power is a very Paladin-like thing to do. I don't think a Paladin of Ancients would worship or deal with Cthulhu or That Which Lurks, or any Fiend.

Exactly one of the three angels has a Flamestrike spell-like ability, in which a pillar of divine fire (half radiant/half fire) hits players. That is the only celestial fire ability, so no, I don't view Fireball (Balors and Pit Fiends have this spell though) or Wall of Fire as appropriate.

Moreover, gaining temporary HP by killing creature and feeding off their life force, doens't sound very "Good" to me, no matter how you try to reflavor it.

My posts reflect how I and many other DMs will feel about that particular class combination, you might not agree, but it bears mentioning because other players will run into the issue.