Ray, maybe my main point got lost in the details: many lenses will work very well with a range of format sizes, up to a (fuzzy) maximum where edge and corner performance declines.
So it is slightly misleading to talk of a lens being for a single format, except through the incidental facts like it having a lens mount used only on cameras of one format. Thus, it makes little sense to say that one is doing 35mm photography simply because the lens one uses is capable of being used with a 35mm format film frame or sensor. To re-iterate, if I were to use a 35mm format lens though an adaptor on my 4/3" DSLR, I would not be doing 35mm photography.
Of course, BJL, you know I've been using 35mm lenses for a long time with a smaller-than-APS-C camera. You might even recall when the first rumours of the 5D appeared causing great speculation on this forum about the details and sensor size, I expressed a hope that it would be the same size as the cameras I already had, the D60 and 20D. In other words, a direct competitor to the D2X at a lower price and with all the advantages of the cropped 35mm format.
But it wasn't to be and the fact that 18 months later Canon is still unable to provide a 12mp APS-C format indicates that the technology is (was) not quite there that would enable them to do this whilst maintaining their low-noise-at high-ISO standards. But who know what they have in the pipeline!
The fact is, I would prefer to use the smaller-than-APS-C format for advantages of weight and longer telephoto reach, but I'm not going to compromise quality for the relatively marginal lower cost and weight. My 5D plus Sigma 15-30mm lens weighs 1.65Kg. My 20D plus Canon 10-22mm weighs 1.25Kg. That 400gm difference would be noticeable when hiking all day in Nepal with a couple of cameras around one's neck. But 12mp on a big sensor are better than 8mp on a small sensor, so I chose the heavier option even though the effective
focal lengths are almost identical for both cameras.
I can only assume that the very small number of Canon EF-S lenses and Nikkor DX lenses available after all these years is a clear indication that the APS-C formats are a transitionary stage. It does not make sense to consistently use heavier than needs be lenses designed for a larger format if a just slightly
heavier, bulkier and more expensive option is available that can provide improved quality. With battery and strap, my 20D weighs just 125gms less than my 5D. That weight saving is insignificant to me, unless it's combined with a lighter lens.
P. S. I am interested, but not overly surprised, by your judgement that the Sigma 12-24 is not very well suited to use with 35mm format. If true, that undermines the idea of a larger format advantage at ultra-wide, since there are good wide zooms for 4/3 and APS-C formats that are as wide as any other other zoom usable with 35mm format, and the reportedly excellent Olympus 7-14 for 4/3 is indeed as wide (114º) as any rectilinear prime lens I know of for 35mm (e.g. the Canon 14/2.. And AFAIK medium format offers nothing wider than the 114º of 14mm in 35mm format.
I should mention that my Sigma 15-30 is pretty good to the edges. This was the lens I used for comparison when testing the 3 Sigma 12-24mm copies in the store.
The resolution fall-off, even at some distance from the edges, was so obvious with the 12-24 (comparing both lenses at 15mm), that I got the impression that this is how an EF-S or DX lens would appear when used on FF 35mm and is why Canon even prevent one from fitting an EF-S lens to a full frame body.
The serial numbers on 2 of the lenses, from memory, were quite close, indicating that 2 of the lenses could have come from the same batch. The 3rd lens, however, had a significantly different serial number.
Nevertheless, it's quite possible that all 3 lenses were the remainders of cherry picking. All lenses, including the 15-30, were equally good in the centre.
By the way, if the term MF photography can include all formats from 6x4.5 to 6x9cm, even though all lenses might not be interchangeable between the formats because of different fittings, I think the term 35mm photography could encompass all the roughly APS-C formats that not only can fit 35mm lenses but are probably more often used with 35mm lenses.