What is the effect of a semolina/plain flour mix in pasta making?
Firstly, all-purpose, high protein ("OO"), semolina, whole wheat and other flours make excellent pastas on their own or in combination.  So, at the end of the day, you should use what you like, have on hand or is economically feasible.  Semolina flour, either by itself, of in combination with other flours, should not impact the flavor much...but it should create a slightly "rougher" texture.  Some claim that this helps sauce cling better, but I have never done a side-by-side comparison.