Canon badly need a decent 'normal' prime for APS-C users because the current offering all have significant weaknesses, as does the main third party alternative. Beyond that, there is a need for some affordable wide angle primes. To be honest, if it saves money they might as well make them EF-S, as anyone that can afford a full frame camera should really be able to afford 'L' glass (and if not, they've probably got their buying priorities wrong!). I don't buy the argument that some people make that it's only full frame users who care about prime lenses. A decent full frame setup will cost you many thousands of your local currency; it's a little insulting to suggest that only the rich can afford to be 'serious photographers'.
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.
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.
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.
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.
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.