From a homeowner's perspective, I can only think of 2 minor reasons to GFI protect the whole subpanel and that's to save a little time or money. I can think of several reasons not to GFI protect a sub.
First, sections 210-8(a)(2,5) have specific exceptions for appliances. The classic example of such an appliance is a freezer. I'd be really unhappy if a receptacle in my garage tripped the GFI breaker and took out a freezer in the basement.
Second, let's say I set up a workshop in either the basement or garage with dedicated circuits for tools/machinery and a separate circuit for lighting. I wouldn't want a GFI trip on a tool to take my lights out.
Third, placing the GFI protection that far "up circuit" from the receptacles could make it more diifficult to troubleshoot nuisance trips. It certainly doesn't seem intuitive to me that an entire panel would be protected. I'd go looking for a hidden GFI feed-through before I thought to check for a GFI breaker feeding the sub (and that's with a GFI breaker for our spa).
