In your hypo, the diamond case, in no way affects functionality. So, yes, both would be identical in functionality.
Nevertheless, it would be the manufacturer putting a premium over non-functional features, eg, diamond coating, over functional features in such an instance.
If the non diamond version costs, say $1000. But the diamond version costs $100,000. Quite clearly, in that case, the manufacturer choosing to make a diamond version is placing a higher value on the form over the function. It is designing with that philosophy where the actual function is quite secondary in value to the form.
So that is the monetary sense of form being more important a driver than function, even if functions remain identical.
Then there is a second sense where form dominates function. The cases, as between the cMP and nMP, quite obviously, are not identical. IMO, the nMP made compromises on expandability and other pro-desired features for the sake of form. Reasonable can disagree on this, of course, but the proportion of this survey, and the comments from the latest macworld article on this topic are telling of apple's success in balancing form and function for the target professional market may be off.
without going hypothetical, there's a real example of what you guys are talking about regarding the mp.
the shell would look sweet when fresh off the lathe.. or a light blast.. maybe more the sheen of a mbp but it would look good after that process.. apple has decided to add some diamonds/bling to the thing by going the extra steps of anodizing and polishing to basically mirror finish.. and you're paying extra for that bling with no added functionality.
however, what seems to be ignored here is that the shell itself is functional.. it's doing a lot more than sitting there looking pretty.. it's cooling the computer.. it's shaped the way it is for a specific reason/function.. it's not shaped that way for people to look at.