Using ARB_framebuffer_sRGB

Is there any reason, other than an artistic one, not to use ARB_framebuffer_sRGB for more correct results? What are the oldest pieces of graphics hardware that support it, and is it free or negligible in its cost of resources across all of them?

sRGB framebuffers are core functionality in OpenGL 3.0 and above. As for the hardware availability of the extension on pre-3.x, you should look at this OpenGL database.