To someone who is trying to make an artistic statement with their buildings, A Pattern Language might be uninteresting, since his advice is essentially the architectural equivalent of composing music in a major key according to traditional rules of harmony. If that's what you want to do, that's great, but if you want to make something deliberately dissonant or challenge traditional notions, it's not so helpful.
It's also worth noting that Alexander has some very strong opinions about things, and (just like any other area of human endeavor) there are architects who disagree with him.
Whether or not his ideas were revolutionary, just going on the way new developments in my city are built, they could really stand to take a page out of his book. Everything is just a big rectangular box plopped down on a rectangular grid with ugly materials. I assume it's all optimized for cost. So right now I don't have much respect for the architecture community.