Meshes in X-Plane, whether modeled in an OBJ, or generated as the results of other "3-d clutter" (road .net files, .for forest files, etc.) can be either one or two sided. So first: two-sided geometry is bad in most cases.
In order to understand why two sided geometry is bad, we must consider the alternative. The alternative to two-sided geometry is to simply create each triangle twice, with one facing in each direction. We can do this in an OBJ without making new vertices - because vertices are referenced by indices, we only need more indices, and indices are cheap.
Thus we have an alternative to two sided geometry, namely "doubled" one-sided geometry.
The first problem with two sided geometry is performance: for a small number of triangles, it is must faster to simply emit additional indices than to change the drawing mode to two-sided drawing on the CPU.
Thus in an object it is virtually never a good idea to use two-sided geometry. That ATTRibute will always be worse.
What about for the other clutter? Forests are currently always two-sided, but that's okay; X-Plane enables two sided drawing just once, then draws a huge number of trees. Same with roads. For facades, there is a cost to using two sided geometry, so only use it for facades that must be two sided, like fences; do not use it for buildings.
Now the second problem with two sided geometry is lighting: X-Plane does not calculate lighting values separately for the two sides of the two-sided geometry. So if you have directionally lit models with two sided geometry, the lighting will look wrong. This is the second reason to use doubled geometry instead.
Things Are Starting To Look Up
There is a work-around to this problem of incorrect lighting on two-sided geometry: "up normals". With up normals, the normal vector for the triangle (which is used to determine how light "bounces" off the triangle) is set to face straight up. The result is a triangle with brightest lighting at high noon, regardless of which way the triangle actually faces.
The good: the triangle looks the same on both sides and has sort of a "flat" lighting - it doesn't look wrong when the sun is setting. The bad: the triangle has "flat" lighting - it looks non-3d.
We use up normals for forests because the forests are made of two-quad trees...the trees look less fake if directional lighting hints don't make the two quads as obvious. You can simulate this in ac3d using the "make up normal" command for vegetation quads you put in your own models.
For roads, the geometry is two-sided, so we use up normals to avoid having the back of a road element look funny. Some day we may do something more sophisticated.
Fixing Facade Lighting
Facade lighting behavior will be changed in the next 950 release candidate. Before 950, facades would receive up normals, always. Starting in 950, facades will get correct normals if they are one sided and up normals if they are two-sided. This avoids artifacts with two-sided facades, but will make one-sided closed buildings look much better.