Like op says, fun and a nuisance. :)

Modular stuff is fun, especially if it looks nice on a shelf, but it becomes a nuisance when your shelf runs out of room, or when you upgrade a system and you either have to re-buy some gear or find that's there's no real replacement.

For example, after buying an N64, would you keep your SNES around just for your Super Gameboy?

I was still playing my NES games when I had my SNES, and I guess I stopped playing once I got the N64, same for the SNES/N64. Actually I disliked the first gen 3d consoles, the lack of details and colors in textures was a large turn off, I never really understood indianapolis 500 on DOS, couldn't stand superfx games and all these games had such graphics.