In the high level TvZ games as soon as mutas come out, terran starts building missile turrets all over the map. Approx, 3-4 per expansion and 5-6 in the main, so that is about 75*9 = 675 minerals ( assuming one expansion ), also there will be some PFs etc. Then how come no one gets this research which is just 100/100.
The only reason I can think of is that it will slow down other upgrades. But I still don't know why this has not become part of the strategy, it is just 80 secs, and people have more than one bays anyways.