I think the idea behind the online dealership is a) to get those cars which you need to complete a certain A-Spec or B-Spec event. I had been waiting for the Formula GT and a German Touring Car for a while although I had the money, and when you have completed everything else at the level you're at, that's frustrating. People WERE complaining about that and it's a good remedy to have that online dealership.
And b), now that these Seasonal events showed up, which are limited to a certain time, it might be difficult to obtain a suitable car for a certain event depending on which Used Dealership cycle you currently are. In the first bunch, you could all buy them in the Premium Dealerships, but to make some variety possible, it's the perfect way to offer those cars for a limited time. That's why they had some older Skylines in there, and now, apparently some RX-7's (I wasn't able to check yet).
So, I think, Online Dealership is all good...