For the backup drives, that's definitely too often and unnecessary. For the main drive, it depends on usage. If you move around a lot of files or very large files often, then defrag should be done more often. However, even then, it's not a big deal. If the defragmenter runs often, it won't have much data to move around and it will be done fairly quickly. Unless it's seriously slowing down your system when it's running while you're using the system, it's not that big a deal.
Erich