Several people mention the algae growing on the structure, but I'm not sure that it matters. Although maybe it does in lakes where you already have lots of structure. But in lakes where there is not much existing structure, fish will locate on most types of structure suprisingly fast.
Hank Parker did a show where he put a big group of crappie in his swimming pool. The fish spread out all around the pool. He threw a rake in the pool - it floated with the rake part on the top and the wooden handle sticking down toward the bottom. The crappie almost immediately lined up right behind the rake handle. No algae involved.
I'm sure not saying the algae doesn't help. It grows on my PVC trees and makes them look more natural over time.
I'd love to see some good research simular to what Hank Parker did. Throw some crappie in a pool or pond - then put a Christmas tree and a PVC tree next to each other at the same depth, then see which the crappie pick. You could go on and on with examples of what to try. Different colors of PVC, PVC with and without algae, etc.