Since I made this thread I think it's time I have my say on this matter as to if these games had never been made.
I'll talk about the story stuff first - i'm sure I would have been left wondering if N really was Ghetsis' son from the beginning, considering one of the sages in BW mentions that he didn't know if they were related or not. And after the sages were arrested by Looker, we had no real idea of what happened to them prior to the events of the second game until Zinzolin, Rood and the other one (I think the one who ambushes you and Cheren in Pinwheel Forest) showed up. To add on to that, it would have still been a mystery to some of us what Alder's late Pokémon was (Volcarona, which is mentioned in the PWT). And what happened to Ghetsis after the events of the first game, that would not have been too apparent without this. But what purpose did he have for giving you the creation trio's orbs?
If they had made Pokémon Grey instead, it would be difficult for me to picture how they would go about with Kyurem and if you'd battle either of its specific forms, which is probably part of why why they made B2/W2 - so they could stay consistent and do something that would actually make appropriate sense with a real showdown with the game's cover legendary, either that or they could have made some different Kyurem form altogether.
And as for the gameplay? While I did love BW in its own right, at first I found it hard to imagine what they would do if they made another game and thinking that they would keep the same region but maybe add some new Pokémon like they did with Platinum. But instead they expanded the region with new locations, including a different starting town and others that were not present in the first games (Virbank, Lentimas, Humilau...), so that was a first for the series instead of recycling. And there were Cheren and Bianca, the former of who became a Gym Leader instead of the Champion, and the latter is the professor's assistant. But ultimately, if these games had not been made I don't think I would have been left being much concerned about the fates of those two. Not only that, but the old Pokémon available in the game did make for the experience and region to not be as iffy. And the post-game, which certainly threw a nice extra helping onto the table.
It was mentioned in another thread, but while having exclusively new Pokémon in BW was cool and all, it would not have been a very good idea to do that twice in a row, because it wouldn't have the same special feel if they were using the same ones. Some people thought BW could have done without the predecessors, but there were some things in it that needed patching and answers (not that that's bad though, I still enjoy them).
So that's my take on the matter.