IMO, the light gem is there only because noone invented better way to provide feedback to players on their visibility.
To be honest I think that the lightgem is there because it was there in the original Thief games, like many other things. That's probably also why we have two lockpicks, although based on the minigame implemented for lockpicking we would only need one.
As said above I prefer the shroud approach of the new thief over the lightgem. It is much more subtle and less precise, leaving some minor uncertainty to the player. But I know that there are quiet a few players who dislike it, although I can't say how many of them actually played the game.
Using a sound meter as a feedback for hearing impaired as stated by HMart is a good point, but I never read any complaints from that side here, so I am not sure we are affected by that either.
This is far from subtle, dude.