orcs have way better kill efficiency in attrition battles (8pt offense vs. 6pt), regardless of personality.
it takes about 5 days of hostile before cleric's sustainability really starts to be a major factor, if people entered war with a proper draft rate and offense:defense ratio. usually by then native economy has taken over, or the war has already been decided for all practical purposes.
what cleric is really good for are races that use their elites to defend situationally - dwarf and human primarily. both races also would want to build a lot of things besides hospitals, while avians really don't have a lot of things they would want to build. avians don't need barracks, don't need guardstations in a lot of situations unless they are stupid about letting themselves suffer hits, are locked out of building stables.
if you're doing avian right you're probably using extended time to land attacks with the rest of the kingdom, and using the speed bonus when it makes sense to do so. fast attacking by itself is worthless, but fast attacking is the best bonus in a lot situations. again, simple question of tactical fail. i would think an avian player would have thought this through instead of thinking "omg imma hit fast for moar honor". Fast attacking to massacre and hit between waves is the best, and in order to do that having the best inherent attack time mod possible is helpful. -25% by itself isn't enough, other tacticians come close to that attack time.
cleric seems like a nice idea on paper and many ages ago it was the best way to go, back when cleric had OP reviving soldiers. like a lot of things, people scoffed at Cleric until BB showed how utterly broken Cleric was, and now that's the new trend the kids are into. i played Cleric before it was cool and remembered being mocked, just as always.