Can I help Seiya with providing some business cases?
1. Some people have multiple Google accounts which they use for different tasks, and they could accidently use the wrong account for my application.
In that case, calling the API returns NOT_ADS_USER, and my software replies that the login has no Google Ads. Those users never, never, never believe that error message. They are sure they have access and show this by logging in to Google Ads - this time with the right account of course. Now, if my software could just mention the user name in the error message...
2. Sometimes customer accounts are not allowed to be linked to an MCC of an agency - for instance a pitch is happening. So a user gets another email address and password. Again, this user screws up when authenticating with the software, and cannot find the account for the pitch. Again, this user claims it's the fault of the software using the wrong login. If only the user name could be shown...
Of course there are more reasons. Just ask yourself why the UI itself always shows the name of the user.
There must be a really good business case for that.