Originally Posted by AaronP
Missing hardware is one reason, definitely, but there are others that include EXPORT_SYMBOL_GPL symbols, system-level problems that prevent the driver from probing the GPUs, and other drivers such as nvidiafb attaching to the GPUs and preventing the nvidia module from working. I'm sure there are other reasons too -- zander would have a better idea of what exactly can go wrong.
Aaron, you are missing one point: if we already have nvidia driver running then there's quite a big chance that a new driver will load too. If we don't have nvidia kernel module loaded then we can modprobe it even if X is running.
So, the question boils down to: can you add an option to nvidia-installer so that we could replace nvidia driver while running X server? The option could be called "--IamPervertAndCrazyIdiot" or something like that. Normal users will never go that way and experienced users will easily cope with the consequences if anything goes wrong.