MoonSong
Rules-lawyering drama queen but not a munchkin
You havent explained why a healer is, in this respect, any different from an archer, an illusionist, a diviner, etc.
If the group doesn't have a healer then that player will be playing a different PC; and it is the contribution made by that different PC (eg in dealing more damage, devliering more battlefield control, etc) that will obviate the need for a healer. My 4e group doesn't have a full-time healer: there is a hybrid cleric (one Healng Word per encounter, compared to the 3 per encounter that a full healer would have), a paladin with 5 lay on hands per day, and a dwarf. But they cope fine with encounters of up to level +5 or 6, because they have other capabilities to bring to bear (eg three controllers via an invoker, a polearm fighter and a control-focused sorcerer).
I don't see why you think the healer role is unique in having to be indispensible to be useful.
As I view it, the healer is entirely party oriented and (in pure form) completely party reliant. All others roles can eventually win fights on their own -ok maybe a pure illusionist won't- if needed, a pure healer (even if supplemented to be buffer too) can't do so, all others can hopefully defeat an enemy, a pure healer at most can stave off death and stay alive but nothing else. By this nature a pure healer needs the party to win fights, and becomes a load if not useful enough. Now the problem is that the line between useful and necessary is thin, and entirely subjective. Ones very useful is another's mandatory. (I consider 5e has no need for a dedicated healer, but a healer is fairly useful, however see how CappnZap holds that the healer is still mandatory)
Your example form 4e is spot on, that is the edition which truly makes healing non-mandatory, but it features no pure healers, nor pure healing/buffing, most of it being tied to riders upon attacking. However much you tell me how useful A healer can be in 4e, I cannot help but feel it is superfluous for the most part, with the edition too geared towards all damage all time.