The main reason I enjoy trilogies and other series is because they are a continuation - which just makes it a long story. I enjoy that because the author then has the ability to take the characters further, entice the readers to bond with those characters and invest in the actual story itself.
On the other hand, I can see where the allure would be for the approach Powers took. I haven't read them (yet
) but I can understand how three books that stand alone but can be connected for an avid reader would be an added attraction - almost like an inside joke only for his fans, not the casual reader.