I don't think it was a complete waste of time. Sure it took too long, and there's really not much that wasn't already expected, and Mitchell seemed to do very little investigation himself (he really just piggybacked the Feds investigations of BALCO, Radomski and McNamee). But putting an "official" stamp on the evidence and testimony makes it harder for people to keep denying that this has been a problem.
A lot of casual fans still thought only a few people did it, or that only home run hitters did it, and that Barry Bonds is scum. This report kind of makes it more obvious to the casual fan that a lot of baseball players did it, not all of whom are people you would consider scum.
I'm glad the report came out and also named names despite the flimsy evidence and the obviousness of its conclusions.