I'm a very active plugin author in the repository and have had many opportunities to help others out using my plugin. Many of the times users have had no problem granting me access to their sites.

I always ask first if it is ok, and then insist they make a temporary administrator account, and follow up with a reminder that they can and should now delete the account.

I find, more often than not, users just pass along their login credentials. Another major issue for debugging options is when the plugin author needs access to the FTP account to confirm file structures or to make sure files are being uploaded into the right location etc.

One time I received a message via twitter asking for assistance, and we got on skype and within 15 minutes we had the issue resolved.

Helpful, knowledgeable and trustworthy plugin/theme authors are a great thing, but I would be very careful who I trust....remember, ANYONE can create and upload a plugin, but not everyone is a trustworthy individual with good intentions.