Hey mods, this is in the wrong section.
Anyway, you most certainly do not have to use PMM to copy songs to the USB drive for Kenwood head units to recognize them. Using PMM creates a database on the drive that makes organizing tracks on the drive, but you aren't tied to the software. You could use WMP or iTunes. You can even just drag and drop the folders/files to the drive, without using any software, though you'd have to take care to organize the files and folders properly to get albums to play in the right track order.
As for the ratings, though, unlike the normal tag fields (artist, title, etc.) the PMM ratings ("My Rating") are not stored in the appropriate "rating" field of the ID3 tag. So, without PMM you would lose those ratings. However, you could use a good tagging program (e.g. Tag & Rename) to manually add the ratings tags.