Ian, they're not "tricking" you into re-buying a game just because they make it available for the umpteenth time. It's certainly in a different format at least.
I would re-buy games, and willingly. I would like to enjoy some NES, Super NES, and N64 games that I already have, even if I have to pay money for the convenience of having them there and not have to set up those consoles again. I'm actually more likely to buy games I already have than those I didn't, which I'm sure to Ian is irrational. But I would still buy games I didn't originally own, too.
Though I do think the price is on the high side. If it was $2 or less for a NES game and they could guarantee every NES game ever made, I could see myself impulse-purchasing so much until I end up spending $1400 on all 700 games or so. Now at $5 a NES game I'll be thinking about what purchases are worth it.