It takes a while for each Android version to make it onto devices. Jelly Bean will likely be no different.
I personally see this as a benefit, not a problem. This way, developers can take their time building in support for new OS versions into their applications. That way, when ICS devices start really getting onto the market in large numbers, most apps will be compatible. The same will be true of Jelly Bean, and all OS versions that come afterward.
You have to remember, not all consumers like being on the bleeding edge like us techies. Most of them aren't willing to sacrifice for the latest and greatest. Most of them wouldn't want a Jelly Bean phone or tablet the moment it came out, because not many apps would support it.
I see this as a good thing. They adopted Apple's New-version-every-year approach with the inception of honeycomb. If anything this keeps new devices up to date with the current Android release while keeping the advancement of the platform steady as well, a win-win in my book.
I know how the business side of things works, but I think that if Google and the android manufacturers and providers would get on the same page with each other, things would obviously improve. Updates would be on time, people would know when their device is going to get an ota. No more people being told " yeah this phone will have such and such for an OS on this month", and no more people getting royalty pissed when it doesn't happen. I know... keep dreaming. It would just make more sense to me. I'm sure there's a billion reasons why that can't and won't happen, the biggest is obviously that the companies all compete for our money.... but I think they could develop a system where the companies could still be competitive with each other and we could have less fragmentation. It would be nice.