Yeah I'm with Rumble, it depends on if you're actually going to use the fold down seat.
They made more standard back seat Dusters and Dart Sports than they did cars with the fold down rear seat, but they also made plenty of fold down seat cars so they're not exactly "rare". As an option I don't think it really adds any monetary value, and because of the additional components used they probably do add a little weight. Not a lot, but some. However, if you're going to use your car to haul stuff, they are VERY useful. I actually converted my standard rear seat '74 Duster over to a fold down rear seat using a parts car, and I think it's a
great option to have. But, I also daily drive my car, and have been known to frequent wrecking yards and pick up fenders, doors, rear axles, etc for my other projects with it. So yeah, very useful. But there seem to be fewer and fewer people that actually use their A-bodies like that, so, I'm not sure I would call it a highly desirable option for most folks.
Here's the "how-to" on the conversion I did on my car, in case anyone is interested. It's not 100% stock, I left the package tray more like a standard seat car.
How To: Fold down seat conversion
And here's a picture of it in use, 275/35/18's on 18x9's sitting on the fold down, 295/35/18's on 18x10's in the trunk and pass through. Try that in a standard seat duster...