There is a reason for this.....
The CUEs are distributed within the release pack because as soon as a ISO format image is renamed, the corresponding CUE WILL need to be replaced. Since the ISO name is included in the CUE as text (and this will need to be updated too) suddenly BANG, you have a new file with a new CRC32/MD5 every time the name is amended. Pain in the ass. So... the CUE files are included for convenience.
GDI/IMG/other format images don't suffer from this. If Maddog for example updates a bunch of Dreamcast game names with a new flag say, the GDI's remain perfectly valid. They will simply be renamed by your ROM Manager/Tool of choice like any other file.