That is VERY old news. And I highly doubt there is any purpose to them other then looks. (There are 3 gates you know.)
Yes and no.

Only the LOD terrain appears in Skyrim, in the same way most of the surrounding landmass was present in Oblivion too.
However, there are details included in this LOD that doesn't make sense unless future expansion will add some real terrain beyond the borders of Skyrim:
-Whitegold tower and Red mountain were inserted in the LOD. Why? There's no point in Skyrim from which this two features are visible. Red mountain would be visible in the distance only if west Morrowind is included (the landstrip between the sea of Ghosts and Skyrim eastern border). Whitegold tower would be visible only from northern Cyrodil or maybe some mountain range in Hammerfell.
-4 roads out of Skyrim. Why? One could argue that Skyrim has to be connected somehow to the rest of Tamriel. When bethesda made Oblivion they didn't include any road to the borders and this puzzled many people.
-The closed gate in Riften. Behind it, there's a paved road, a few secondary locations and a gate to... west Morrowind. Why that gate was closed in the first place? There's a road behind it and a few locations that can be accessed by traveling around the mountain. That closed gate alone is, for me, the greatest hint Bethesda could drop about the future DLCs.
-In addition to said road to Morrowind, there's another leading to it past Refugee point. Not one but two roads to Morrowind!
-The road southeast of Falkreath leading into Hammerfell. Again, only for the sake of completeness.
-The road and closed gate to Cyrodil. Well, this one may remain closed. it's the place where supposedly, the player was caught trespassing and thrown onto the cart with the rest of the prisoners being sent to execution. Its presence just makes sense from a histhory point of view.
-No roads going into High Rock. That's interesting... Bethesda, for whatever reason, created roads going into the neighbor provinces but not to High Rock. There's a road, west of Solitude, that leads to nowhere, just like the roads of Oblivion once they reached the border. A hint that High Rock won't appear in any future DLC planned, maybe?