In case you should need to reinstall the OS for whatever reason, I have more peace of mind knowing that data is on a different drive entirely. It can also save time if you’re upgrading your system you don’t have to download games again, for example, though you could just bring an old drive into a newer system and likely achieve the same, but to me it’s simpler to have it separated, and then you don’t have to worry about booting from the right drive. Which probably answers your last question, no I do not dual boot but if I did I would probably have at most 1 drive for data and 1 drive per OS (though most likely I would just partition a single drive for OSs).