I'd much rather have a FF with crop options in-body. This should be standard, like choosing file quality.
Please forgive me if I am missing something obvious as I've never even considered this....but, other than file size on your CF card, how is this different from simply cropping a full frame image in post?
There are two main reasons for the APS-H format. One is that it's substantially cheaper to produce than FF (APS-H is the largest size that can be imaged in one shot onto the silicon wafer during production, FF sensors require multiple passes). The second is that the smaller sensor size allows a faster frame rate - dual Digic IVs could not achieve 10 fps from a FF sensor. But, if an in-camera crop is applied, a smaller portion of the sensor could be used, allowing a faster frame rate. So a FF camera with a fast shutter could achieve higher frame rates that way, e.g. Nikon's D3 is 9 fps in FX mode (but that's only 12 MP, so a lot less data to manage than Canon's current 21 MP FF), but edges up to 11 fps in DX mode (1.5x crop).