Absolutely, but I don't think that the OP was asking about the sort of forms that UK dive clubs use, or commercial dive op requirements around the world under different legal systems. I think that the questions about the form that PADI required before a class.
If this was just about safety and not about liability, then why will every instructor accept my signature on a form? I'm an ENT doc, I'm absolutely not qualified to clear someone for diving with heart disease, asthma, diabetes, etc... Has anyone ever encountered a situation where an instructor gets a form with a "yes" box checked, looks at the signature and then decides to vet that doctor to see if he or she is qualified? I mean, I guess that BlueTrin's story about the Thai doctor is sort of like that, but that sounds like just one person's decision, and it wasn't a PADI training issue.
AFAIK, that doctor's signature is a binary thing - you have it or you don't. If this was about safety and not liability, the agencies would have a list of doctors who were certified to be able to clear people for diving. And even then, you might need different subspecialty clearances if you had more than one problem. Coronary stents and vocal cord paralysis? What kid of doctor would be able to sign off on someone with both of those things?
The doctor's signature is a way of transferring some degree of liability away from the instructor (to the doctor) in a training environment. That doesn't mean that medical evaluation and a frank discussion of the risks involved with diving for any given condition isn't crucial - see the excellent explanation of that by
@rsingler upthread. But the agencies and instructor's insistence on a docs signature on the form isn't necessarily for that.