In reply to Gaston Rubberpants:
> Their significance is dependent on the grade you climb.
This, to me, is the key. I've lead a couple of HVS and seconded a few E1s, but I am fundamentally a VS-and-below punter. To me I can distinguish between VD, HVD, Sev, quite easily - and, probably, MS too (although perhaps the difference between HVD and MS is more 'feel' thing than a difference in absolute difficulty). And I see the difference between HS, MVS and VS very keenly, because that's more or less my limit these days.
Hard Diff, on the other hand, I'm not sure I believe in! At that level there are just so many ways of doing the problem that trying to grade for one of them is a bit arbitrary; I think that's the same argument that makes it reasonable not to bother with tech grades below 4a, that you may well just not be climbing the same sequence. At 4a and above there is, likely, a 'best' way of doing it - within the tolerances of height, strength, etc.