Well, The diffusers I use are Face Hosted, so I cannot place them in mid-air without a Reference plane to host to. I also don't have to "Align" and "Lock" the diffuser to the Plane.
I see you had to use the "Connect Into" to get it to work properly. Is that the only way? In previous versions, I had no problems with this. when you connect duct to a diffuser (manually), it should remain connected, even when you move the diffuser. Now, you have to tell the Revit you want it to remain connected, even after you physically connect it? What is the reason for the change?
I've been using Revit MEP since version 2008 (with formal training) , and Architectural since 2004 (my 10 yo. nephew could figure it out).
Please let me know why you have to use "Connect Into" to get the connection to stay.