I think it's just something Blizzard does. Catering to those people who like to explore. Take Durnholde for example - all of Hillsbrad was available for exploration - and all the NPCs were era-consistent, it was pretty cool if you're into that stuff.

Way back before Mount Hyjal came out (I'm talking pre-BC, not just BC-before-Hyjal) the whole zone of Cavern of Times was off limits. If you managed to get in (by wall-walking or getting feared behind Nozdormu) you wouldn't see a blank area, it was filled with psychedelic colors and designs, it was kinda cool.

You could get into Caverns of Time simply by dying in front of the doors and rezzing on the other side. When I went in there it was everything but the walls, floors and NPCs. The "doodads" like the broken ships and stuff like that were there. Just nothing else. lol

Another example is the portal outside of ZF. Mages can (or used to) be able to blink through the portal, and explore much of Tanaris sans mobs, etc. It goes as far east as Gadgetzan (which is just really a placeholder model), as far west as the slope going down to Ungoro, and goes as far south as the area with all the pinchers coming out of the ground... Literally 60-70% of Tanaris.