I'm actually curious about this subject since I've never really encountered another summoner who's done this. There's something I've been doing, even since 75 original cap that I wanted some input on.
In more difficult fights, I find it very irresponsible as a summoner to rely on simply one avatar until it is defeated in fights where there is no threat of them taking hate. Some rage blood pacts of another avatar are more effective than others, same with wards. So I fine myself quick summoning and releasing the avatars I need based on damage of rages and buff/debuffs of wards.
For example, fighting VWNMs like Kaggen, it buff with attack boost which can be very detrimental. So I'll summon Shiva to Heavenly Strike (using Mana Cede before if I have the chance) then summon Fenrir to use Lunar Roar to dispel. Although in the case of VWNM such as Ig-Alima, I use Diabolos' new blood pact which has a 100% effective dispel where in normal dispel methods, even Fenrir's, does not work.
Does any one rotate avatar for rage and wards respectfully?