Kipman
I know this is months down the road from this entry but it is as close to the debate I'm into now. I hope you get alerted.
2.21 in D1.1 states "Memebers in tubular STRUCTURES shall be Identified as shown in Figure 2.14." In this figure it clearly shows that a T, Y, K connection is between intersecting members of HSS shapes, these are types of "tubular connections" that require specific considerations. With your theory all connections would be a T, Y, or K which in my opinion is ncorrect. You may have "tubular connections" that include "T-joints" that are not "T connections for tubular structures", such as a baseplate on tube steel or a tube member welded to the top of a wide flange.
To further understand the requirements, AISC helps to clarify the conditions. In "HSS Welding Requirements in AWS D1.1" (section 8 page 24) you find these statements, "apply to welded HSS to HSS connections" and "while Part D deals primarily with HSS-to-HSS connections,"
Look into these.
You rely solely on a definition (tubular connection) to establish your theory, but find the definition for T, Y, or K connection and rethink your opinion. Also the whole concern in the industry is dealing with the "welding with out backing" issue in this type of connection. You cannot get adequate backing in place due the geometry of the connection. We need not "read" to much into the requirements, keep it simple.
Please consider these references and responed, this has been an ongoing question through out our industry. I have seen it go both ways, and those with the "authority" usually win out even if they may be wrong.