Ext2 would be preferred because we definitely don't need any journaling capabilities if we're not constantly writing to the disk.
With a quick look at the source code, it seems that WBFS does NOT fragment its files, so there is no need for a defragger. It does this on purpose, because the data for the file needs to be contiguous when loading it. Possibly, there is a need for a compactor, though. (Smush all files together to eliminate gaps of free space.) But the problem is that keeping contiguous files can result in wasted space:
In this case, a user has 4 files on his hard drive. He deletes file B. Then he adds file E. Space is wasted (unless he adds a file that can fill that space, but this is never a guarantee).
Regarding corruption of games, maybe there's a problem checking free space, which would cause this problem: Just a wild guess.
I don't feel like looking into the source in depth right now, because it's 3 am here; goodnight!