Not sure what specific group of photographer is really looking for FF and high fps ? Sports photographer ? Why would they crave FF ? Landscape/architectual ? Why would they need high fps ? Wedding/model photography, I would think they would appreciate FF, but 5fps should be plenty fast enough.
And your point exactly, those who want high fps may not like the large file size, so 1D3 is probably good enough ?
Seriously, I see D3 facing competition from 1D3, which are almost on par from MPix perspective, same 14bit, 10fps vs 11fps, 1.3x vs FF. For those pro with tons of white lens, I doubt D3 is a value proposition enough to switch. Similarly, 12MP vs 21MP, that is a gap. And you may say D3 high ISO, 25600, that sounds astronomical, but it's merely 2 stops above 6400. And Canon had been known to be conservative on their ISO max. Let's see the actual output from 3D before any judgement.
D300, not sure what is all the big deal other then the 51 AF points.
But I guess these 2 are good enough to give a knock on Canon's head, to make a good 5D2 for me next year