He's 29 years old. Injured though he may have been, his stint with the Grizzlies only ended because of a behavioral issue, not because of an on-court problem. I give Larry Brown all the credit in the world for taking chances on guys who have supposed behavior problems, because most of the time, I suspect said problems are more an issue of fans feeling bad (causing management to panic) than actual problems between teammates.
When Miles did play recently with the Grizzlies he picked up right where he left off in 2005-06, and in some ways was better. Granted, it was in under 300 minutes, two seasons ago. The problem with him is that everyone thought he was going to be Tracy McGrady or Kevin Garnett, when all along he was more like Stephen Jackson. Had Miles been drafted in the second round, he might not be getting these chances, but then again, he might not have been saddled with crushing expectations from the outset. The fact remains that he was drafted third overall because he was a superb athlete, and he's getting these chances because decision-makers see his body's potential.
What kind of player is he, anyway? In short, think of him like Jax, but with PF/SF swing abilities instead of SG/SF. He used to soak up offensive possessions when he shouldn't have. His career PER is below average because of one monumentally poor season with Cleveland and his final season in Portland. The rest of the time, he hovered around average, and as he got older was showing some signs of gradual improvement.
More relevant to the Jax comparison, though, is that it looks like Miles was a very good defensive player his entire career. We don't have +/- for his Clippers years, but Basketball-Reference and 82Games both agree that he was a disaster in Cleveland and something of a revelation in Portland. In fact, he may have been the Trail Blazers' best player in 2004-05, coming off the bench for a team that went 27-55. A lot of that had to do with his defense, as Miles simply doesn't have the skill to shoot or get to the rim particularly effectively, but his length and quickness did seem to translate to the defensive end. Think Jamario Moon with somewhat better overall offense.
At this stage of his career, there's no way any team should hand him a rotation spot, and his knee should be thoroughly checked out. However, I wouldn't be surprised at all if Miles signed with the Cats and became a solid rotation player. In a best-case scenario for Miles, this squad would be Don Nelson-ized with a Larry Brown twist. Imagine a lineup of, say, Shaun Livingston, Stephen Jackson, Gerald Wallace, Tyrus Thomas, and Tyson Chandler, with Boris Diaw, D.J. Augustin, Gerald Henderson, and Miles coming off the bench. Augustin's the only one you'd call a real shooter, but everyone but Chandler can step out of the lane to shoot. Everyone but Chandler can put the ball on the floor. Everyone but Augustin and Diaw is supposedly a plus defender, and Diaw's not bad at all... It's unlikely, and there are better ways to build a winning team, but that might be the most exciting path to take.