I haven't done the fight, but in general there are many fights where there are periods of time where healing is low enough for it to be covered by one less healer, thus enabling smite spam, this is usually true for the beginning of the fight when everyone is 100%. Also you usually cast holy fire every 10 or so seconds, so you can activate arch angel and boost your healing. Naturally you shouldn't be casting it doing very intense healing periods, but it's hard to determine when that period is unless you've done the fight a few times, and if you just assume that all periods are intense, then you never cast it and thus lose out on better healing.