Exactly. Equally useless, therefore balanced.
It seems that we are using different definitions of "balance". I have a rather strict one, and it goes like this. Decide what quality you want to measure, and what objects you want to measure. Measure them, and compare the difference(s). The less of a difference there is, the more balanced they are in that respect. This entire time I was measuring "DPS" between "healer jobs". If the healer jobs all did 20 dps, that would be balanced. If they all did 3000 dps, that would also be balanced.
As such, let me clarify and repeat that dps balance amongst healers (or tanks, or dps) is important, but does not bring value on its own. You mention jobs being useful, which does bring value. Amusingly, this is done right now by making sure damage bewteen healers, tanks, and dps are unbalanced in favor of the DPS, which you agree is a good thing. This imbalance is a choice made when designing the jobs, and these conscious choices make all the difference in the world. Things like "usefulness" are a bit nebulous though; I'd argue that role usefulness is unbalanced in favor of Tanks at the moment.
Still, regardless of balance or imbalance, what's most important is how players feel about a job. Usefulness is a part of it, yes, but so are other intangibles like aminations, rotations, glamour, story, so on, and so on. That was what I was trying to impart - there are many things that go into this, and not one of those things are king. Enjoyment, or rather, the combination of it all is king. And the thing is, you can't exactly measure that, since each person values things differently.
You mentioned earlier that Balance brings in the business. I believe certain imbalances hurts business, like dps within the healer role. But what brings in the business in the first place has little to do how jobs stack up against each other.