I haven't been debating the ethics here. certainly your statement re: legal =/=ethical is true, but there's little room for debate as to whether an action is legal or not. it is or it isn't (within a given jurisdiction). wrt one particular action, however, reasonable minds can differ as to whether it is "ethical" or not. it's very subjective
as for "advantages," I suspect that most people do not know how to kill themselves properly. initially it seems that it wouldn't be so difficult a task, yet many methods are messy and if one is unsuccessful, one might end up worse off than he was to begin with. wrt suicide via some type of drug, most people don't have the practical knowledge to determine what type/dosage of drugs will successfully complete the job, nor is it always easy to obtain the drugs. all of these things certainly are considerably more difficult for a terminally ill patient. this is where having a physician prescribe the lethal dosage confers an "advantage." also, there is a stigma associated with suicide and it may be argued that the stigma is lessened if the law sanctions the suicide