FILM CRIT HULK has another very good take on the issue here:
Unlike the Writer's Store link, though, he doesn't argue against acts at all, but rather for dividing a story into as many acts as the story requires (5, 9, 12, whatever). His basic argument is that an act break happens when the main character(s) makes a choice that sends the narrative in a new direction, and as such, 3 acts is actually way too few to make an engaging story.
It's a interesting way of looking at it, and hearkens back to Maurice
's comments elsewhere that fan films tend to have passive protagonists who don't act to move the story forward. You get a bland, unstructured mess that way - even in professional films. *COUGHGREENLANTERNCOUGH*