When you keep my previous observations in mind it really should not surprise you that on the model you used as an example, which is both really light geometry wise and has lots of big planar surfaces, the effect of the wireframe slider would be minimal.
Like I stated before its goal seems to be to reduce visual clutter and presumably at some point in the past, also to increase performance.
So you are most likely working from the scenario where you are talking about models whose polygonal density is so great they almost turn solid when viewed in wireframe. Objects that are scanned at a high resolution are a good example of such a case.
Lets take the Stanford Dragon as an example this time and see the effect the wireframe slider has:
So in specific cases it seems to do its job quite well.
Then again, I am not really arguing for keeping or removing it. I am personally indifferent to it, that scan is not particularly heavy for current hardware and I seldom work in scenes which get so cluttered I’d need to optimize my wireframe views.
That said other people have expressed its useful for them so from that point of view it seems valuable to keep it. Perhaps it could be renamed so its intended purpose is a little clearer.