Still, keep in mind the mountain bike companies also know that upgrading people from hardtails to full-suspension bikes is a good way to keep sales going. Plus, dualies invariably will weigh more than a hardtail with similar frame/components. There's also some small sacrifice in pedaling efficiency, but not much these days.
Look at it this way: I just bought a new mountain bike, and it was hardtail. Steel, too-I mean, how retro it that? But, I didn't think my riding (moderate single-track) called for dual suspension, and I wanted the weight-savings or a hardtail. Besides, steel in part obviates the need for dualie because it's such a great-riding material. The bike is Pine Mountain from Marin Bikes. Lovely bike-Columbus tubing, mix of LX, XT and XTR components, Answer Mars front shock, disc brakes. I've been extremely happy with it. And price is not bad: $1,300 for a bike that should last years and has a frame that's well worth any future decision to go all-XTR or something.
And yes, I likely will retrofit a suspended seatpost. I had one of these on my last moutain bike, and have one on my tour bike, and think they make a ton of sense. Not quite the bump-soaking capability of a true rear shock, but light and moderately effective. I like the USE suspended posts, which go for $130.
In short, I think a front-suspension only is fine for the majority of rides and riders out there. Dualies are great too, but there are tradeoffs, and you want to make sure you can justify one.