it's not "a little extra mp cost"
pa'agrio's honor requires twice the mana
also, ritual of life has ~25% more power, so it's great to keep specific targets alive
not to mention that OL skills in classic are clan skills and not ally skills and if the party is scattered for a second the use of pa'agrio's honor might be the true waste of mana. we also had only 2 cps in the clan at that specific siege.