I'm not a doctor, so I'll leave that for doctors to rationalize. I did a quick Google search, and I gather that there is a lot of controversy over the actual translation from the Greek of the Hippocratic Oath, and that there are many different versions being administered to young doctors. Hence your question may be relevant for some doctors and irrelevant for others.
From a practical point of view, this will lead, as it already has in Holland, to "non-voluntary assisted suicides" (euthanasia)...
"Slippery slope" arguments tend to be suspect, and need to be scrutinized very carefully before being considered valid justifications for supporting or opposing laws. Certainly I (and I believe most people) can see a rather distinct difference between "voluntary" and "non-voluntary" assisted suicides. Nor does the example of Holland carry much weight, whatever may actually be happening there. There is obviously a vast cultural gulf between the United States and Europe, as exemplified most recently by Euro-appeasement in the War on Terrorism and in the upsurge in Euro-anti-semitism. Things that happen over there provide extremely weak evidence (if any at all) to support a slippery slope argument for this country.
Because anyone can commit suicide now, the only point of assisted suicide laws is to lend some social, moral respectability to suicide.
Please. I doubt that "social or moral respectability" is the foremost consideration (or even in the top ten) for most people contemplating suicide. "Anyone can commit suicide now" only if he or she is physically able to do so (which is often not the case for the terminally ill), and if practical means are available (terminally ill patients are frequently unable to jump off bridges or gain access to guns or obtain lethal drugs on their own). The main "point" of assisted suicide laws is to allow a patient to voluntarily die in a relatively painless manner, not to seek societal respectability.