I've often though about the same question and came to the conclusion that when a procedure is qualified using a grooved joint only tensile strength is determined. Yield strength and ductility (reduction in area or elongation) are not usually determined and are not typically included as criteria for the acceptance or rejection of the welded test piece. That being the case, the tensile strength of the weld serves as the basis of the allowable stess on the weld.
The base metal in the other hand is tested and ultimate tensile strength, yield point, and ductility are typically determined as part of the testing regiment. As you noted, the yield point is used as the basis of design calculations for the base metal, but tensile strength is used for sizing the weld.
Best regards - Al