Just because "other daws" have had a feature for years, doesn't make them small features. And unless you have a crytsal ball and can predict the future, we have no way of knowing what we will or will not "ever see again" in the future as far as effects or instruments go.
It seems perfectly logical to me, for the Props to offload the job of creating instruments and other devices to third parties via Rack Extensions, thereby allowing them to focus on the core of the program, workflow, etc. making it the best it can be.
Plus, REs mean we can pick or choose what new devices we get. Maybe I didn't much care about Kong because I was perfectly happy with Redrum, or maybe I do all my drums only in REX loops so was perfectly content using the OctoRex.
I personally prefer the notion that I can select only the devices that *I* want in my system, rather than paying for devices I might not really care so much about. For instance, if R7 included Rotor, Radical Keys, and PX7... it would also probably cost more and personally I don't care about Rotor, and am not that excited by even PX7. Radical keys I like very much but at this point have chosen not to pay for it because my piano needs are covered in other ways. I have the option here though, and options are good.
To me, this is a smart long term move for the company and ultimately gives the users the best of both worlds - talented creative third parties who will come up with more and better devices than the Props, and with much greater frequency - and a core company who can deliver the best DAW/Sequencing platform available. I'm very optimistic about the future of the program and I feel that Reason 7 is really a turning point.