I disagree with your statement regarding FF users and the cost of an FF setup. FF users are not much more likely to be rich than your average prosumer APS user these days: here in Canada, a 5D2 is $1999 and a 7D is $1449, which is not a huge difference. Also, used 5Dc's and 1Ds(II)'s can easily be had for less than $1500.
Your entitled to disagree, but don't forget that the 7D itself it quite an expensive camera that most photographers can't justify spending out on. I take your point with used full frame, but I think the age of the cameras you mention is a put off. Not only do they lack a lot of the features that even the Rebel line has these days, but I would also be concerned about spending a significant wad of cash on such a old camera; these days cameras are basically consumer electronics and as such, I would not trust their reliability long term versus a film camera (maybe that's just me). By the way, a used 1Ds MkII still sells for more than a 7D, here in the UK at least.
Secondly, a 5D2 with a 35/2 provides better performance at a cheaper price than a 7D with 24L (my set up); similarly, said 5D with 50/1.4 is much cheaper and better than said 7D with 35L. You can pretty much go on forever with similar comparisons. A 5D2 with 24/2.8 isn't even comparable to APS, as an EF-S 15/1.8 doesn't exist and if it did, the price would be frightening.
Sure, but a 5D2 with a 35mm f/1.4L will perform even better; are you suggesting that you should shell out over Â£1500 for a camera and put a Â£220 lens on it (I'm sticking to my home currency here)? You're right when you state that there is no comparing the 5D2 with a 24mm lens to an APS-C camera, that's my point -Canon haven't bothered to produce a fast 24mm equivalent prime for APS-C. You believe that a 15mm f/1.8 for APS-C would cost a fortune; I'm not sure that I'd agree, wide angles crop lenses require a lot less glass than for full frame, which is the whole point of EF-S (but I think that a 15mm f/2 would be a better performance-price compromise).
If you look at it this way, you can see that your statement about a "decent" FF kit being expensive is quite wrong. One (myself included) might even say the opposite is true. I suppose that makes me look dumb, as I have a 7D/24L, but in my defense, they were purchased years apart, I need the 7D's features and my mkI 24L was way less than the new one is.
No need to defend yourself, your main camera requirements dictated that a 7D was the better choice of body for you. The fact that you had to shell out for a 24mm f/1.4 to get a (roughly) 35mm equivalent fast prime demonstrates that there is a demand for dedicated EF-S lenses in this range. How many people haven't bought a 24mm f/1.4L because it's too expensive for their budget, but would buy an EF-S 22mm f/1.8 (for example). Now you are thinking about buying a 5D MkII in addition to your 7D to meet your ndesire to use fast wide angle primes (I'm guessing -am I correct?). This would mean that you've covered all bases, but it's hardly an affordable option.
If you don't need high PD or the AF/FPS of a 7D, how exactly are your priorities wrong in buying a 5D2 with cheap glass? You get better performance for less $$$ and often end up with a lighter, smaller kit as well. There's no rule stating that FF users are stuck buying L glass and honestly, they need it less than us APS guys.
First, what if you do need both (like you?), buy a 7D and a 5D2? Second, how much do you think that a 5D2 be worth in ten years and how will your 24L be worth? Third, you keep insisting that it's a 5D2 or a 7D; most people shoot with Rebels and XXD cameras.
This is why I like the idea of Canon updating these non-L primes: they would benefit both APS and FF shooters. These lenses are mostly fine optically, all they need is USM, more aperture blades and maybe a tweak here and there. They should definitely be kept EF though, even if there is a small cost savings to make the wider ones APS, there's more than enough interest in them from FF users. Personally, I'm looking to add an FF body as soon as I can afford it and would definitely buy a 35/2.
If Canon could do this, it'd be ideal. I just think that, especially with wide angle glass, there are quite a lot of cost savings to be made by going EF-S. Compare the price of the Nikon AF-S 35mm f/1.8 G DX to the much poorer spec Canon EF 35mm f2.0; or the Canon EF-S 17-55mm f/2.8 to the EF 16-35mm f/2.8.