Well from my walk today, I visited about 7 flower beds at the university. Each a good 2 or 3 blocks from each other.
One flower bed had a trap, the rest did not. From my observations, there definitely were plants that the bugs LOVED, enjoyed, would be on, and didn't care about. If the plants where in the bed, so were the beetles regardless of the trap status.
For each of the beds that had the plant that the bugs LOVED and no trap, the plants were very damaged and I collected the most from that bed. The bed with the same type of plant that had the trap didn't really have that many beetles around on the plants, but the trap was filled with thousands of those insects.
If I have time next week, maybe I'll go around with a camera and try to do more of a scientific study for you guys.
I think the traps are worth it. Unless you're out and about in your yard every day, ready to squish each one you find, the trap can do the work for you. Sure the trap is going to attract them, but the plants they want to be at are doing the same thing. But don't put a trap out if you don't have a problem, then you're just inviting trouble.