I mean...the other healer's kinda right, though.
You're asking for an extra spell for no reason other than having a second button to press. No interesting interactions, no skill expression, etc.
Quite a few people want (and many use add-ons to make) the 1-2-3 and 1-2 combos on DPS and Tanks to be single buttons. So a lot of people don't, in fact, want "a ------- basic combo chain", as people are actively using tools to turn those into single buttons when they can get away with it.
Don't get me wrong: In my longstanding position on Healer changes, I do think having a Healer Job or two do that is fine - I've even proposed SGE have a MCH-like 1-2-3 combo chain - but giving it to every Healer just because you want convoluted extra buttons to do the same thing isn't likely to be a winning proposition.
Some do, some don't: See above.
Pretty much.
Right now and in the past, the answer was that there were different Healer Jobs and people could use the one that more appeals to them. For example, AST has a Card side-game to distract oneself with (imagine if Cards had only 1/3rd their effect, but Draw had only a 10 sec CD...). This is good because some players want to have more to juggle and do to not get bored.
The trick is to have that gameplay exist alongside the other for those that do not.
Honestly, disagree.
One of the things that so annoys me on DPSers is that they, the Jobs/Role whose entire purpose is to do with damage, mind you, all have basically the same 1-2 + Job mechanic AOE rotation. MNK has a 1-2-3 + Job mechanic, but is otherwise the same. The Tanks have the same thing as well. It's so homogenized and it's not even interesting. It's even led over the years to a lot of big attacks (like Ruin 4 and Summons, and even RDM's Holy/Flare/Scorch/Resolution) being made AOE because people just want a single target ability that also works in AOE.
It's like BLM's with Dispair/Flare and Xenoglossy/Foul. While some types of players (BLMs) like this, most others...typically find it less than appealing having one button you only use for AOE and one button you only use for single target that otherwise do the exact same thing. There are probably plenty of WHM's that wouldn't mind Holy being Glare or whatnot (single spell that does both AOE and single target). Often times, the AOE rotations for Jobs are just slimmed down things that are less interesting than the single-target ones anyway, so people would rather do their single target rotation in AOE as well and just have the attacks be AOE to begin with.
It'd one one thing if the AOE rotations were all distinct...but they really aren't. So having two things that do the same thing is kind of boring to a lot of people, kind of like the difference between Glare/Glare and Glare/Water doing the same damage but being two buttons vs just one.
What's better is buttons that do different things, not buttons that do the same thing just in AOE.
...and like Ty says, WHM actually has the most DPS gameplay of all the Healers right now. It's always weird when common perception and reality meet and don't agree, but this is one case where that's true. Like the "Healers: Then and Now" thread looking at how Healers were actually played, how their kits have actually changed with time, and the realization that SCH is the worst "Boilspam" of the Healers, not WHM with Glare, which is actually the least-bad offender of 1-1-1-1 compared to the other three. Pneuma isn't really a separate spell from Dosis (same damage, just AOE I guess), and is on such a long CD that it's not really part of the standard rotation anyway, and Toxicon is gated by having only 3 to start the encounter off with and only getting more at a damage loss by casting shields and having them break.
We aren't playing Vanilla WoW. AOE doesn't happen in most boss encounters (or is VERY short and adds are often where your AOE rotation is only marginally beneficial, if even beneficial at all), and MOST of the AOE rotations (not all, but most) are just budget "we have single target at home" versions of the single target rotations.
As for RDM specifically - because it isn't a Tank or Healer and has nothing else to do with its time, but sure, whatever. I have thought before RDM might be more enjoyable if we removed the procs. And it is obnoxious that its AOE rotation is the reverse of its single target rotation for god only knows what reason. The 1-2-1-3-1-2-1-3 gameplay on RDM is nice since it has nothing else to do. Removing the 4/5 I wouldn't be against, though. I guess you could remove Stone and Fire. Or make Thunder/Aero single target and Stone/Fire the AOE while Jolt and Impact are the shortcasts. Though I suppose Aero/Thunder would make more sense to be the AOE longcasts and Fire/Stone the single target ones. Jolt-Stone-Jolt-Fire and Impact-Thunder-Impact-Aero...yeah, that would be fun, I think. Then melee combo. Also be nice if there was a way for the AOE to also be 50/50 instead of 60/60 for, again, god only knows what reason. Just have it be two hits for 25 or something, I dunno. Or three hits of 15 each and make the AOE combo also 45/45, whatever.
When people want to play DPS Jobs, they play DPS Jobs. When playing DPS Jobs, you have nothing going on other than pressing damage buttons (and RARELY something like a mitigation). That's how the Trinity system works.
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I do agree with Semirhage, though: 1-2-3 is fine for a few Jobs, but shouldn't be standard on everyone. Especially for Casters, RDM or SMN type rotations work better.
I wouldn't mind, though, if Fire -> Thunder or Stone -> Aero for RDM when you have Dualcast. It seems somewhat unnecessary because RDM has such good hotbar economy, but I wouldn't be opposed to it. What I'd kill for is them to make the AOE rotation Impact->Thunder->Impact->Aero, though. It being opposite the single target rotation just makes me irrationally angry.
As for the 1-2-3 - MCH also has this. SGE having something like that with lasers would probably work. My past proposals for Chemist were for it to have a 1-2-3 combo with its Caster Gun weapon, like MCH.