Although I believe trout are opportunistic feeders, I do believe they are taking the Hopper pattern as just that.
Think back to when we were kids, (That's a long way for some of us), when we went fishing we used worms and when we could catch them, live hoppers. I don't think the fish were mistaking a live hopper for a Stonefly, and they did catch fish.
On the other hand I don't think the trout key on Hoppers as much as they do on Stone's so maybe an overlap.
My $.02
BK
Then again, who really cares. Fishing hoppers is just so damn much fun