I think people are underestimating the value of another shot blocker to go alongside Garnett. The idea that you can always have two shot blockers on the floor is going to be huge if Miles can get healthy. I believe that is also why they brought in POB.
Miles block shots, but he's often out of position defensively to do so. Blocked shots aren't indicative of good defense, and I doubt teams are going to be overly intimidated, even should Miles become completely healthy (which isn't likely.)
I agree, Perk is a great defender but not a great shot blocker, he needs to protect the basket. The nice thing about a SF who is a shot blocker is that you don't expose the basket. You may be vulnerable from outside; but is nice to be able to help from the weak side. Concern about Miles' ability to recover or get back into position with his injury are an issue though.
IMO, the best thing about having a SF shot blocker, especially in Team Defense, the opposing player always has to watch the weak side as that SF could be coming from behind to get a nasty block(pinned against the backboard). Good examples of players like this are Josh Smith(mostly PF), LeBron, Tayshaun Prince, Odom, etc.
Darius Miles, given the hope to be healthy again, can contribute greatly with just defense. The part of the games last year I hated most were when the Big 3 sat down for a breather, and 1 or 2 of them were forced back in b/c the 2nd unit would blow a 10-15 point lead (House/Cassell, Tony, Posey, Powe, Pollard)etc. Having at least 1 big defensive contributor with some 7' bigs down low, I think we can hope to see the Big 3's minutes go down at least a couple points per game