That's reason why it's cheat protected and should not be changed.
No, I don't think so. CVARs like r_gamma already make things brighter, so there is no reason why r_ambientscale shouldn't be unlocked. Try it out and see for yourself.
Because ambient scale only applies to entities, not the entire picture. So players and other things glow, but not the rest of the map. (Personally I think its a good idea, and brightskins )
Saying that they "glow" is an exaggeration, but it makes them stand out a bit; which is good, because they are a bit too dark compared to the rest of the textures on most maps.