I noticed a interview with Canon from the CP + in Japan.
http://www.megapixel.co.il/english/archive/28040
"Q: The new 24mm and 28mm seem to be really perplexing, they are not extremely fast, surly not cheap and have image stabilization which doesn't seem to be that necessary in such a short focal length. To top this you have the 17-55mm f/2.8 with IS which admittedly isn't a prime lens but can give you much more flexibility – so who is the real target audience here – videographers?
A: Videographers are indeed an ideal market for these lenses, but the main group who prompted the design of these lenses where in fact photojournalists. While a f/1.4 lens is desirable, they do tend to feature quite large optics. So for example in situations where photographers are trying to blend in and be less obtrusive, a lens with a smaller diameter is more suitable. We included IS to help these photographers to continue shooting in low light, even with smaller aperture that these lenses offer. It’s also worth noting the original versions of these lenses are nearly 25 years old. When designing their replacements we wanted to look forwards to ensure that these lenses enjoy a life span to match their predecessors."