Will Azelf see any use since Mewtwo exists?

As it stands, no, it really doesn't have any use. If you are an active raider and accumulate natural Azelf candy, no harm spending them to get another psychic attacker if one is needed, but there's no reason to put RC into Azelf when Espeon and Alakazam are so close in performance and Mewtwo is a better RC-reliant psychic attacker.

Unless PvP changes something, I don't see much use for any of the lake guardians as none of them will be best in class. Azelf (the high attack one) is bested by Mewtwo as a high DPS psychic attacker, Uxie (the high defense one) is bested by Lugia as far as bulk goes, and Mespirit (the one with more balanced stats) is very middle of the road in all regards.

If you are doing a 6 unique raid then it should be required.

Alakazam > Espeon > Azelf > Gardevoir > Sky Attack Moltres > Mewtwo for Machamp or Hitmons should be fine.

It's a more expensive, *slightly* better Alakazam. Not worth it unless you're going for unique species challenges.