I think there are a lot of different reasons here for emulators and against them. A very healthy conversation.
I have only used an emulator many years ago to play mega drive games. It was a chance to play games I missed out on when I was young. Some I didn't realise existed and had to play. There were a few classics I wanted to enjoy again like Zombies Ate My Neighbours, Paperboy, Cool Spot, Surf Dog Alien, Animaniacs and Splatterhouse.
Some of which I never owned and only played at a random friend's a couple of times.
I think they are a somewhat good thing, as mentioned by others some would of never been enjoyed by people. Also the whole money/space/hardware thoughts from Spacedeck makes a lot of sense.