Store metadata in song file, not in some app data folder

This has been discussed time and time again. The two sides are:

PRO:
- Yay, I can have all my details in my files, and never lose them if I wipe my library.
- It may eventually be easier to transport files between programs
- Can be implemented as a plugin/option

CONS:
- Significant problems on Multi-user systems. If you share your music directory with your teenage sister, you will notice a general trend in your library towards "high school musical" type songs.
- Changing a rating/play count may use considerably more resources, because the file has to be rewritten.
- Greater risk of data-loss. If the power cuts out whilst writing a rating, you most likely lose your MP3 file. As you will be writing to them now every 4 minutes, it increases the chances of corruption
- Changes the MD5 signature of the files. There is a lot of free music out there shared on torrents such as
- Wont work with portable music players anyway, as there is probably no set standard.
- Wont work with multiple active players well. They will keep changing details on each other.
- There may be other ways to store the information that doesn't touch the files, but can be easily shared.
- Doesn't like Read-only files, so potentially unreliable operation.

Overall, I think this method has many cons, and it would have to be optional anyway (with a lot of warning or people will start whinging). However, you have just given me an idea of how to do this idea, that is incredibly efficient, totally safe, easier to code, independent of file type (no chance of slightly funny tags shonkying stuff up), and lets everyone win on this idea.