1. In the early arranger days we had no audio recording capability on the keyboard only midi so the easy way to listen to anyone else’s music was to copy and play play the midi on your own keyboard. Nowadays it is still sometimes done for a few reasons eg if you want to play along, change the key, or learn the song at a slower speed, because midi files can be edited easily.
2. If someone makes a style for a song you are at their mercy whether or not they did a good job on it, provided all the parts or not. So free registrations and styles are a very mixed bunch, sometimes good , sometimes terrible. And sometimes a song specific style listed by someone is no more than just a standard style where they has set the tempo for that song. However, if they are commercial ie you paid for them, they should be good. But again, not always true.
3. As I have said above, you never know, but most companies providing these “packaged products” commercially do a decent job, otherwise they would soon be found out and not sell much. Free stuff will seldom involve the fake book or music. You have to work that out yourself.
Overall, it’s the internet, like everything else there are some gems out there, and sometimes it’s free, but there is also some (quite a lot?) of junk to sift through. You can only find out for yourself. I have personally never used anyone else’s registrations, because I find that I prefer to pick my own styles, voices, etc for a song. But many people like them. If you want a style that sounds a lot like the original recording, and the person has done a nice job, they can be great. But frankly, 90% of the music you will play can probably just use standard styles. You are just as capable of picking a suitable voice and style as anyone else. And you learn best by doing just that. At least that is my opinion.
Mike