I'm not trying to speak for anyone here but myself. But part of why I play the games I do, and the way I do, is out of nostalgia. I'm nostalgic.
Nostalgia, 2: a wistful or excessively sentimental yearning for return to or of some past period or irrecoverable condition; also : something that evokes nostalgia
I'm running The Keep on the Borderlands partly out of nostalgia. I chose to run GURPS Dungeon Fantasy partly out of nostalgia - a return to a simple dungeon-bashing game instead of the bigger, wide-area and (largely player-driven) plot-and-problem-centric games we used to play. I have all my old gaming stuff (well, almost all) partly out of nostalgia. I can look back on this stuff, even stuff I won't use, and remember why I got it, how exciting it all was, etc. It inspires some nostalgia; it's natsukashii (懐かしい), even, for you fellow Japanese speakers.
I say "partly" because while nostalgia helps drive my choices, it's not driving my games. I could have chosen B1 In Search of the Unknown (which was my second dungeon ever) or B3 Palace of the Silver Princess (either the first or second module I ran). I considered B3 but decided it was more work and less fun than B2, but I considered it. I'm playing the 4th edition of a 2nd or 3rd generation RPG, depending on how you count generations, not going back to the systems that were my introduction to gaming. I'm using old modules but not ones my players have been through before, and I'm using a new-to-me megadungeon, something I never tried to do before.
If we weren't generating new fun with this old stuff, we wouldn't keep doing it. My games are a mix of old stuff I miss, new stuff I wanted to try out, and old stuff I never did get to try out. I would keep playing if it wasn't adding new value to my life, but it's not like I went back to my old D&D stuff from when I was 9 because of a purely forward-looking outlook. I went back because I miss that sense of wonder and that simplicity of play, amongst other things. My gamers show up and play partly out of that too, I expect, but again, if it wasn't new fun who would keep showing up? I'm creating new entertainment out of a mix of old and new.
I think it's fair to say that nostalgia can be a positive, if it's what brings you back to something old that you used to enjoy and can enjoy again. Pure nostalgia - nothing new at all, enjoyed purely as remembering previous fun - doesn't seem so useful. Doing bad stuff or boring stuff or sucky stuff because that's the way you used to do it - eh, I can't do that. I don't miss a lot about my old D&D days. I like a lot of the changes in RPGs, and what drew me to GURPS from AD&D and Rolemaster. But the bits I miss, well, they helped keep me in fantasy gaming all this time and helped me decide to go back to an even more simple campaign than I'd been running. I got a little tired of a big sandbox with an over-arching plot (a plot jointly generated by my actions and those of the PCs) and realistic consequences for violence. I wanted a little sandbox of a dungeon with PCs returning to a tavern and selling off the armor and weapons of their fallen foes.
Like I said, this is just me. Other folks may play old systems and old style games for other reasons. I'm playing new systems with old style dungeons for a lot of reasons. These aren't even all of mine. It's just the occasional negative tossed at nostalgia that made me think about my own relationship with that feeling.
I think I play partly out of it, and I don't think that's a problem.