You do not want to use the render meshes exclusively (if at all).
Back in the days of Quake 3, its curved surfaces (quadratic bezier patches) were evaluated at load-time using a "Geometric Detail" user setting for tessellation quality. If collision were done against those surfaces it would have killed CPU performance. So instead brush planes were used, they approximated the curvature of the surface several orders of magnitude simpler for the sake of collision.
Doom 3 did away with this, and evaluated curved surfaces at level build-time... there were several reasons for this, but it meant that collision geometry was once again relatively static. Modern games are back to the point where patch-based surfaces are evaluated using varying degrees of quality at render-time (DX11/GL4 tessellation). You cannot possibly hope to match your collision geometry against what is being rendered on screen when it is dynamically tessellated (you also would not gain much by implementing such fine-grained collision), so that actually necessitates separate simplified collision geometry.
In fact, gameplay often relies on collision against things that are never actually rendered. There are collision meshes to keep AI from taking certain undesirable paths, to prevent players from running off into the distance, and so on. You will probably never get away with simply colliding against the rendered geometry in a reasonably sophisticated game.