Rondo,
WELCOME TO THE AWS WELDING FORUM!!
After saying that, take this as my own morbid curiosity:
1) I was hoping you would join back in and clarify some questions raised by Al, but, as you have not;
2) I find it had to believe you have even met over 15,000 CWI's in order to use the phrase "MOST" (emphasis mine) which would imply more than 50% of the currently active CWI's certified by AWS. Is this like, "4 out of 5 doctors prefer" or "recommend" this product? After asking 100 doctors and finding 4 who will step out on a limb and endorse the bogus product in question they throw in one more just to make it look good. Unless of course you meant, 'many/several of the CWI's I have conferred with about this issue interpret that to mean...'. That might make a little more realistic sense.
3) Due to the location of the quoted text I presume this is the 2010 edition as the 2006 and 2008 have it worded different and listed under 5.24.4 I believe.
4) So, as such you want to specifically go to Table 5.9 and 5.10 and then Figure 5.4 (D) which gives you 'Desirable', 'Acceptable', and 'Unacceptable' for Groove Welds in a T-joint. The illustrations really make it pretty clear for interpreting the text.
5) To understand your question any further we would really need a more clear statement as to how you are viewing this 90° angle situation but it is not a code specified or explained profile.
6) To answer what I perceive as your question ( your last sentence but with no question mark after the text) bluntly, NO, a 90° transition with no reinforcement is not REQUIRED (emphasis mine) to fulfill 5.24.3. Especially if by 'reinforcement' you are referring to an additional fillet weld following the filling of the groove up to the flush condition as illustrated in Figure 5.4 (D) and then an added fillet weld from Figure 5.4 (E).
7) The majority of the text involved here is referring to Groove Welds in Butt Joints. However, it still applies to T-Joints when viewed in context. 'Reinforcement' is referring to the extra height of weld material above the plane of the two plates joined by a Groove Weld in a Butt Joint. The "Welds shall have a gradual transition to the plane of the base-metal surfaces" phrase is then referring to the shape of the weld 'reinforcement' as it transitions back to the plane of the plates being joined so that there is no stress riser from ropey beads with undercut, overlap, etc. It still applies to the transition of the T-Joint as well but with different application which must be understood from Figure 5.4 (D). This can also be seen by referring to the Commentary on this section to see what they are dealing with.
8) The joint design is established by, and then in shop drawings, approved by the EOR. As such, we don't know the stresses, vectors, etc that the EOR has built the joint to handle. We don't need to second guess and tell the fabricator that they need anything other than what has been approved by the EOR. Just look at the drawings with a comparison to Figure 5.4 (D). If there is a question, NEVER refer to MOST CWI's. Always use the RFI procedure to find out what the EOR wants. CJP T-Joints are used for specific purposes where a standard fillet weld will not transfer the stresses as required. But, that doesn't mean that the joint needs a weld that flows across the joint as the fillet weld would in order to "have a gradual transition to the plane of the base metal surfaces".
He Is In Control, Have a Great Day, Brent