I've noticed when checking into properties in Tokyo (ex: Hyatt, Hilton, etc), they generally want a copy of your passport. This seems pretty standard practice in many other countries as well, although I am still not quite sure exactly why. Showing a drivers license or passport doesn't generally seem to be a requirement at hotels domestically in the US, where a credit card seems to do the trick.

Anyhow, I plan to venture to Hakone soon and will be staying at what I can only describe as a "traditional Japanese hotel" booked by a friend. I have little more than that to go on. I was planning on paying in cash and not planning to bring my passport. Do you feel that may be a problem? Will some form of ID be required? If my friend has their ID (Japanese citizen), will that be sufficient?

I realize it's likely difficult to answer specifically for this property, but generally speaking, what are your experiences and thoughts?