I can't speak for any other posters, but let me be absolutely clear. I'm not advocating simply lying to get around the form. What I'm saying is that if one does his due diligence, and reviews his physical condition with his regular physician, who then renders a knowlegeable medical opinion that there in no contra-indication to diving, than whether he lies on the form or not is immaterial, since nothing will have changed either way.
Unfortunately there are others on here who did imply that. The RSTC form states the following: "Have you EVER had or do you currently have" (emphysis mine). If the student ever had a condition listed, and the student and his/her regular physician (who is more than likely NOT a specialist in diving or barotrauma) decide that it is a non issue and the student puts down "NO", then he/she has lied. This has been upheld in court.
As for the instructors need to know, that too depends on materiality, and in any case you can't have this both ways. On the one hand, instructors say they quickly scan and file the form and MD letter without reviewing the details, aparantly saying they don't actually need to know specifics.
Since the instructor has the final say if someone can take his/her course, they had better know the specifics.
On the other hand, most instructors, unless confronted with a glaring problem, aren't qualified to render any medical opinion, which is why divers are told to get an MD letter. If dive professionals took it upon themselves to render a medical judgement, they'd be buying into an obligation and possible legal liability.
They can contact DAN and speak to a physician who is a specialist in diving and barotrauma and get a second opinion. This way it is the specialist who renders the judgement, not the instructor.
Considering I've contacted some of the physicians and personally explained the possible medical consequences to some of the conditions their patients marked off, yet were cleared to dive with, it's interesting how many of them don't even read the form either. (woman 1 month pregnant for instance) Because of my call, several have revised their clearences from fit to dive to not fit to dive. The title MD doesn't mean they don't make mistakes or don't get lazy. This is why it is the instructors duty to understand the possible consequences of the different conditions mentioned on the form and why the instructor has the final say, not the MD.
So, we're back to square one, If you are fit to dive the medical form is simply a legal release. If you are not fit to dive, stay out of the water. If anything on the form gives you pause, get a qualified professional opinion before signing up for a dive.