In DQ you'd essentially get other people's characters in your game's Inn rooms. The more people, the more stuff is unlocked, including item recipes you have no other way of getting.
I don't doubt that it works great in Japan, but you have to walk to a specific place in the game world to put it in sleep mode, then it's just sitting there draining battery and you can't play it on public transport while that's happening. I'd imagine the hordes of people playing DQ in Japan actually play DQ, and don't use that weird sleep mode. With real StreetPass, you could exchange characters in that program, then the info could be relayed to the actual game without the very specific situational requirements.
I also feel like StreetPass seems so amazing only because it's placed in the context of a system with very lacking online features, but that's another can of worms.