I usually run 8 or 9 man Avenger squads. I'd run them 10 strong, but I turned 2 of my 10 models into exarchs. The only purpose I see to minimum Avenger squads is to spam wave serpents with objective secured, and that always ruffled my feathers. My large avenger squads (and even their exarchs) always do me proud. With a little luck, you can get a scary amount of rending out of them. That 4+ armor makes a huge difference over guardian armor, and a seer tends to compliment this unit very well. Also, I've been using my exarch to turn my avengers into a tarpit unit these days. It isn't as effective as it was back when we had defend, but the exarch's powersmake him a wonderfully annoying way to shut down power klaws or special characters in other squads. If a farseer is in the neighborhood with Fortune, he can make your shimmer shield reasonably impressive. And then the assault units can descend, usually in the form of allied incubi.
Avengers are great. They can scathe most units in the game that aren't AV 11+ vehicles, they punch back in melee just hard enough to make your opponent know he hasn't assaulted pushovers, and they can pin the enemy down in melee with a little luck.