I don't believe this has been suggested before.
IIRC Jorb and Loftar think it ruins the fun factor in caves/mines that people can just use a client to see in the dark.
Everyone uses it... because it is available and they would be at a loss if they weren't using it. (I use it)
If they really wanted to make it so people would still use the mining helm and torches they could try this.
When you enter a cave, in that little area that is still lit up, make a status affect pop up called "Scared of the Dark" or something to that affect.
When you equip the mining helm or light the torch in your hand the affect will disappear and let you continue on into the cave.
Obviously, without the the light source you could not move into the dark area.
I know this wont fix the fact that you can still see the whole area of the cave if your using the client, but I guess it will force people to use intended features of the game.