The main advantage of MBOX files is that you only have one file per mail folder and all the filenames are known and constant no matter how much mail comes in. I

The main advantage of MBOX files is that you only have one file per mail folder and all the filenames are known and constant no matter how much mail comes in. It means if you have a ton of mail you aren't dealing with massive numbers of tiny files. The files are still plain text and contain the bodies and full headers for each message so I can use grep or even perl to pull information from them easily. Exporting into other formats can still be tricky though because not everyone can handle MBOX and there are variations.