Requiem Angel is a 5/5 flyer for 5W with a decent effect. Whenever another non-Spirit creature you control dies, you put a 1/1 white Spirit creature token with flying onto the board. For 6 mana, this is certainly OK. She gets better alongside Herald of War, also from Avacyn Restored, who can reduce the casting costs of your Angels for each +1/+1 counter on her. This means you can cast Requiem Angel for as little as one mana.
Also, Requiem Angel's ability to create tokens is especially good in the wake of a boardwipe. Each of your non-Spirit creatures that dies will net you a Spirit token, as long as the Angel dies at the same time. So, Requiem Angel is good protection against boardwipes.
Interestingly enough, while Requiem Angel does still see play today in Commander, outside of Radiant, Archangel Angel Tribal decks, you don’t see her alongside Avacyn or other Angels, including Herald of War. According to EDHREC, Ghave, Guru of Spores is the Commander she’s paired with most. This seems a strange pairing until you consider that Ghave decks produce a lot of tokens, especially Saprolings. So, when any of those Saprolings die, especially when sacrificed using Ghave’s ability, you get a Spirit token.
Teysa, Orzhov Scion has also recruited Requiem Angel to create the 3 white creatures she can sacrifice in order to exile a target creature. Token-happy commanders such as Rhys the Redeemed, Trostani, Selesnya’s Voice, and Darien, King of Kjeldor also co-opt her services on more than a few occasions.
Another Commander who takes advantage of the Requiem Angel is Jazal Goldmane. This does make some sense, considering that Jazal can boost the flying Spirit tokens the Angel creates. It makes even more sense when you consider the Angel was re-printed in the same Commander 2014 deck as Jazal.
Requiem Angel never really saw Standard play, which shouldn’t be too surprising due to her position on the mana curve. But she’s great in EDH decks that can take advantage of the tokens she creates. While she’s hardly any sort of staple, this Angel is a good one to have around.