One file for several reasons. First, it minimizes the amount of file system "slack" space (space wasted at the end of every file, since files must take up increments of the file allocation size). Second, it's faster to open and read/write just one, or a few, files than hundreds or more.
What happens when it gets corrupted? If any jpeg should fail to decode, the tile is marked bad and it will be downloaded again. Additionally, the reload maps tool can be used to forcibly delete and re-download a tile (if for some reason the jpeg decoder is fooled into thinking it decoded correctly).
Also, the map file does split at about 650MB to allow for easy archival to CD-R.
