All SKY textures end up with only a poly count of 6, no matter how messy that sky textured wall might look... same thing as it not being there, almost.
The only downfall will be the BSP file size getting larger because the number of Vis Leaves increase... but not your w_poly count... if that makes sense.
Not to mention your compile time when VIS tries to run.
Like Playbus said... VIS prefers nice 90 degree angles, not sloped / angled lines. If anything, use 45 degree angles... like the ones you use on corner of walls.
Hope that helps...