[*]Games are programmed with some backward compatibility, they will run older versions, but only so much. Guild Wars is an example of a DX8 fallback with DX9 features.
Okay, I re-read your post, I do agree somewhat. OpenGL gets those features by using some CPU power I suspect.
For example: Draw buffers/multiple render targets are in core OpenGL 2.0. But you don't necessarilly need an OpenGL 2.0 card to get them.. you just need the GL_ARB_draw_buffers extension which can be available, and accelerated, on a card/driver reporting 1.5.
Long time ago I remember running Need for Speed 3 with OpenGL in software mode, and it just was not possible to play at all.
If an OpenGL card isn't 2.0 capable, it'll still run at 1.4, 1.3, though you will not get the GPU features of the higher level.