In reply to no_more_scotch_eggs:
> Is this as much a 'cultural' debate as anything else?
Possibly, yes. When a physicist encounters something like Occam's razor, they immediately start thinking: Is it true? If so, why is it true? Can we extend and generalize it? Are there some deeper ideas behind it?
By thinking along those lines physical scientists have arrived at a more general version and understanding of Occam in terms of information and probability. We still call it "Occam's razor" as a nod to the past, in the same way that we use the term "Copernican Principle", though we could call it the "principle of parsimony" is people preferred.
Indeed, in nearly all cases in science the developments of the past (Galileo, Maxwell, Newton, etc) are nowadays stated in refined and developed formulations, rather than their original. To a scientist what is important is the insights and understanding something led to, not the original formulation.
The opposite of this approach is exemplified by philosophers such as Tim, who stick with a literal reading of an ancient text, ossified for all time. They endlessly discuss those ancient texts, but are not seeking to make progress and leave them behind. (As I said upthread, Tim reinforces the less complimentary attitudes that physical scientists sometimes have about philosophers.)
[The irony being that the specific wording that Tim regards as "Occam's razor" was actually a development and refinement of Occam stated 300 years later. What Occam actually said (among other things) was: "For nothing ought to be posited without a reason given, unless it is self-evident (literally, known through itself) or known by experience or proved by the authority of Sacred Scripture". Obviously we're not going to use that version, are we?]
Now, when it comes to specific tasks, such as diagnosis in medicine, a rule-of-thumb narrower version might prove useful. And not invoking extra diseases without good reason is sensible. However, I submit that this version as you expound it is *not* faithful to the original and is indeed less faithful to the original meaning than my version!
Occam (and the other medieval philosophers who said similar things) were asking whether an entity existed at all, not whether a particular instance of a known entity existed.
For example, if you know of the existence of a beach-full of sand grains, then postulating extra sand grains for another beach is *not* a violation of Occam. That is not what "multiplying entities" is about (it is instead about postulating new and hitherto unknown types of entity).
Thus, if it is known that certain diseases are common and prevalent then suggesting that a patient has more than one of them is *not* a violation of Occam's razor, it is actually quite parsimonious, in that it doesn't go beyond known expectations (though of course if you can explain the symptoms with only only one common disease then you should).
So I think that your version -- while it might be useful to you in practice -- is not that faithful to the original meaning.