Some of them were pretty memorable, like Duplica (though she came back in Johto), AJ, Giselle, and the hippie with the Pokéflute to name a few. They were probably better developed one-shot characters, but there were those whom you could tell were just made for that episode, and thus flat as the cel they were painted on (the brothers in the Eevee episode, the samurai and the biker punks, for instance). As a kid, I believed in Ash when he said "one day we'll see each other again".
I was such a fool.
Do I wish they would return in a future episode? Yeah, a few times whenever they came to mind I wished they would show up again, but it only happened at least three times when the plot called for them (Suzie, Todd and Duplica are the ones I can think of at the moment). Even then, my expectations for them weren't really as great, and it slowly made me realize that one-shot characters should just remain as one-shot characters unless the writers could give them a proper re-introduction and keep their characters the same, but add a little more development to them. That's why I feel Duplica is probably one of the few one-shot characters I really liked in the whole series. She was rather unique in that she was a copycat (probably based off Copycat from the games now that I think of it), and in her debut episode, they used her copycat talent to thwart Team Rocket (and poke fun of them), which you don't really get to see much in an episode, a Chekhov's skill--especially one that was a character's signature trait.