In openings where the kingside reaches the following position (the ones that I have been playing are KIA, KID, and Pirc), do you especially value the light square bishop moreso than normally?  I am just getting started with these openings and in most games I will move him if it allows me to play a tactic, and will trade him away for a decent positional advantage.  I have found that he oftentimes becomes a bad bishop late in the game.

 

But I have heard some players say, never under any circumstances lose this bishop.  What is the consensus here?  What do the masters do?  Is there a rule of thumb when it is acceptable to lose him or not?

i post a puzzle how to destroy this kind of defence..wait when my connecting current is better.

If this bishop goes, the light squares around the king are weak. If you trade it for your opponent's light squared bishop, then only his Queen can control those squares. (Knights can occupy them, but not control other squares of the same color at the same time).

 

As such, I assume that one general guideline is that it's OK to exchange light-squared bishops if the Queens already have been or also get traded off, otherwise the trade benefits your opponent  more than you.

If your opponent can (somehow) swap that Bishop against another piece, then there's the possibility for a LOT of trouble along those light squares.

This is called putting the bishop in fianchetto (meaning flank), although many people are starting to use it (wrongly) as a noun. Anyway, it is good for the bishop as it controls the long diagonal, and the king is fairly safe. However the bishop should not be traded off if the queen or bishop of the same colour are on the board as they will use the gap in the pawns, created by you, to attack your king.

Yes, I understand "finachetto".  Yasser Seirawan and someone else use the term "Kings House" to describe a king castled behind a fianchetto.

Several games I have traded off my bishop for another minor piece + positional advantage (IE, trading it for a knight in a closed position, trading it to double pawns, trading it to break an outpost).  I dont understand how it makes the position weaker than a non-fianchetto kingside castle with unmoved pawns?    

I'd like to see how to destroy that defense easily!  I've done it...roughly.  But My best idea playing against it is always..."Get that Bishop the Hell outta there!"

Your last question is easy to answer.  The strongest pawn formation around the king is 3 unmoved pawns.  Not only do these pawns have the option to move at any time to guard against threats to your king, but they dont provide any targets, and they nicely control all the squares in front of them.  Should any piece, say a knight try to attack the king, then one of these pawns can simply take it. 

 

Now if you move the g pawn forwards one square, and trade off the light-squared bishop (or simply have the bishop elsewhere), you have broken up your nice pawns in front of your king, and you have locked them in place to a degree.  Any further movement of these pawns is going to create serious weaknesses.  Already the f3 and h3 squares are completely unprotected, and provide excellent attacking opportunities for knights.  Queens and the opposing bishop can also use these squares if they are still on the board.  For example, stick a bishop on h3 and the king is stuck, with no easy way to remove the bishop.  Follow this up with Qf3 and it's game over.  Also, if your opponent has a bishop or queen along the long diagonal, then it now penetrates right into your king's position.  If they can get both on this diagonal then you're in big trouble, since you have to defend mate threats on multiple squares.  Essentially lose your fianchettoed bishop, and you are giving your opponent all sorts of attacking options which you will have to guard against, since he can basically use these squares around your king as he pleases.  Even in a closed position, i would not swap this bishop for a knight.  Also, you should be aware, that once you commit to this structure, your opponents will likely have the plan to trade any of their other minor pieces, for this bishop.  You probably shouldnt do this too willingly.  It is just a general rule though.  Sometimes it can be ok to trade this bishop, but be careful.  Don't do it without thinking about it first.  

Thanks guys, some very informative posts here.