If you trust the individual to be your onsite host, and basically run the space day to day, can't you trust them with the login? I handle day-to-day operations for a friend's listing, and we even shifted the login and primary email to mine. Mainly because she didn't want to receive all of the email from guests (and potential guests) that I was dealing with anyway. But I can understand how in some situations that might not work.
Regardless of how you work this out, one important thing to do is to add the other individual as an "authorized contact". We had forgotten to do that and it caused some delays in a direct-Support situation.