By default. I didn't have any choice in the matter. With the dongle plugged in it was all that was shown in the "Host Controller Selector" window.So your dongle only took precedence after you took care of that with the Bluetooth explorer or using the bluetooth explorer you noticed it takes precedence by default?
...and the dongle at the bottom of that page is a picture of that GBU421 I am currently using.Here's a guide to the plug-out procedure that I had bookmarked:
http://oliverwolfson.com/apple-mac-pro-bluetooth-issue-solved/
Incidentally, originally when I was playing around with and without the dongle with pairings things seemed to get confused with failed pairings of the BT devices. And I was having more trouble waking the dongle out of sleep. I found the most clean way to set up the pairings for the dongle was to at least once use the Bluetooth Explorer's "Modify Software & Device Configuration" (from menu Utilities) and do a "full factory reset" (leave the first four boxes checked and do the "Perform actions") and then reboot. After re-pairing the dongle behaved much better waking up and seeing the mouse. But as I said in a previous post it still can fail some of the time.