Just like the tittle asks, is it?
I will join those who say, 'No'.
It is clear that individuals can teach themselves how to dive. There are, in fact, several very well known dive 'professionals' who only gained a formal certification after many years of competent diving, and only then because they wanted to be sure that they didn't provide a 'bad' role model.
The question, as put forward, should NOT be answered on the emotional basis of strong opinions about bad instruction, IF this is really a thought experiment.
Resources to help people understand the basics of scuba diving are abundant, they are readily accessible, and competent, motivated individuals can use them to get to the same point as divers at the time of Open Water certification.
The certification 'process' - the instruction - may expedite learning. In fact, I personally believe it probably does. But, that is true with many endeavors.
Some of the discussion has raised the issue of liability. That is reasonable, but not necessarily as applicable in scuba as it is in flying an airplane, or driving a motor vehicle, as two examples. If I do something stupid underwater and die, the immediate impact on others around me is frankly trivial. (And, yes, I am making that statement while acknowledging the potential risks to others who may have to (attempt to) rescue me, etc. But, in driving a motor vehicle, if I am incompetent, I may well harm other people driving, other people not on the road but off it, I may cause material damage to property, etc. The same is true with flying an airplane.
Now, having answered Eric's question as I have, I don't want my remarks to be interpreted to say that there is no value in the current certification paradigm. It is coupled with training, and 'certification' is a tangible outcome of training. Much scuba training is good, much is valuable. Some is mediocre, some is downright bad. Some is unnecessary. The presence of 'some' mediocre' or 'unnecessary', or downright bad' training is not a reasonable basis for concluding that certification is unnecessary, even if it may have prompted the question. There are other, more logical conclusions to be drawn, instead.
To address a suggestion made at the end of the OP, I see little / no value in an 'independent' certification process, because I don't believe that 'certification' is necessary to begin with. At the same time, the notion of 'mentors' is, to me, very positive. The notion of 'coaches' (I am not saying this to specifically praise UTD, by the way) is also very positive. We have gold coaches, tennis coaches, financial coaches, life coaches. Why not scuba coaches, for those that want coaching, and want to pay for it?