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I see your point, but on the other hand, I guess I embrace a little bit of the Field of Dreams approach, i.e. if you build it they will come. This hasn't exactly turned out to be a truism for Android OS devices, though. I just did a little search online, and there are still NEW devices being sold that are running 2.2 Froyo! Not many, but a few. That is a little absurd. I looked 2.2 up because until fairly recently that was what was on my device. I gave up waiting for the manufacturer/carrier to push out an update, and it still hasn't happened, so my lack of faith was justified. The silver lining with Android is that you can root the device and install an updated OS yourself, generally increasing the useful service life of devices beyond what they would otherwise be. Obviously that isn't something that the majority of users are even going to attempt though, and to be fair, they really shouldn't have to.
With respect to Jelly Bean being an incremental update, in light of the low percentage of devices running 4.0, it wouldn't make sense to push out a significant revision, but if you can fix/improve a few things without making life hard for developers and ensuring backwards compatibility with 4.0, go for it. Hopefully that will be the case, anyway.
One thing that is tricky for manufacturers is that carriers are always wanting some "value added" skins/themes/apps pre-installed and generally impossible to remove without rooting, so that an update isn't just one simple software release, then you are done. And frankly, carriers don't seem to care very much about getting out updates. After all, most of them have already got you by the throat for a two year contract, so what are you going to do about it? They would rather you be locked into a device that is frustrating to use by the end of your contract, so that you will be essentially forced to get a new device, along with a new two year jail term, er, contract.