When it's possible to run homebrew code, such as emulators, on a handheld, this in most cases also means you are able to boot downloaded images. That's the main selling point really.
I bougth my Dreamcast and PSP for homebrew though, I made sure that I got myself one of the old fat PSPs so that I could downgrade it to run emulators. Same for Dreamcast, I knew it would run all my old NES game, so I bought it. Best purchase ever.
As for the emulation of handheld consoles on your PSP: That's only for those who really have no money, or loser kids who are just too desperate to invest something in their own hobby. I don't think it affects sales too much, emulation, especially of handhelds, is just too underground for that.