I don't want to derail the thread too much, so I will be brief (Note that the following analysis works in exactly the same way in a multihealer environement):
Consider a player with L HP, taking D DPS over T seconds which will prove lethal.
Let healer 1 stack an absorb buffer A before the damage happens and then provide H1 HPS (heals and absorbs) during the T seconds. The total HPS provided by healer 1 is A/T + H1. In order to prevent the person from dying (D - H1)*T - A <= L (1)
Let healer 2 just produce H2 HPS with no absorbs. Then in order to prevent the person from dying (D-H2)*T <= L (2)
Notice that the value of absorbs to survival is inversely proportional to T.
Since the condition for survival is not maximal HPS but rather conditions (1) and (2), absorbs become indispensable to survival only if condition (1) holds, while condition (2) does not hold. It is immediately obvious that since H1 < H2 for disc as the absorb healer, absorbs become indispensible to survival only for small values of T, because H1*T and H2*T and their difference increases with time, while A is pretty much a constant.