I've got nothing to add.
Yes, If you have only one tab (or window) open, use the close window shortcut. for xfce4-terminal, that would be CTRL+SHIFT+Q.
This is a bug since you have to memorize two shortcuts while in most other applications (not limited to Konsole - KDE terminal, Mozilla Firefox, Google Chrome and Internet Explorer) the last tab closes the entire application. You may close this bug report for as many times as you want while I will reopen it as many times until this bug is resolved. Logically it makes no sense to have an extra shortcut to close the last open TAB. Yes, it shut downs the whole app but unless you're mentally retarded that's totally expected.
I'm sorry for being somewhat cocky but I'm filing all these bugs here 'cause upstream is more or less dead (or dormant if you don't like the word "dead"). Perhaps I will write a patch myself but I don't want to get into the innards of GTK coding for the moment. Besides I'm not a programmer.
Upstream isn't really dead, Xfce just moves at the pace it does based on people who work on it's spare time. This upsteam report is at lease very related: https://bugzilla.xfce.org/show_bug.cgi?id=10691 I'd suggest adding your comments there. I'm sure they would entertain a patch. I don't think we should change behavior of this downstream only. Feel free to re-open, but I don't see us doing anything here until upstream lands a change.
That bug is very hard to understand and follow - perhaps that's why it has received zero attention. I've filed a new one. I won't reopen this bug since I have to agree that it makes little sense. A patch from you would be nice though.