18 ( +1 | -1 ) A B on g3/g6 could easily be attacked by a N moving f3-h4/e5 or f6-h5/e4. The B would either have to move again or be exchanged, doubling pawns. A better defensive setup is play h3/h6 and have the bishop on h2/h7.
17 ( +1 | -1 ) Little to fearIf your opponent's King is also castled kingside, it would be inadvisable for him to open the h-file by exchanging on g3. You could then utilise the open file by playing Kh2 and Rh1.
222 ( +1 | -1 ) Like most things chessical, the value of a bishop on g3 or g6 depends on other positional factors. One point that has been well brought up by philaretus is that a Knight on f6 or f3 can usally harass the Bishop from h4/h5 or e4/e5. So if a player is going to place his Bishop on g6 or g3, he must consider the value of his Bishop and whether he would accept an exchange with the opponent's Knight.
Take as a premise that your own King is uncastled and your opponent's King is castled king-side. If your opponent exchanges on g3/g6, then hxg opens a rook file on his castled king, and that rook can coordinate with the unexchanged QB to attack h7 or h2. It is generally the case that it is *unwise* for your opponent to make such an exchange until you have castled Kingside as well, which is justified by practical experience and the obvious folly of opening a rook file on your king's position.
In my chess experience, many players over-value Bishops vs Knights and will easily make such an exchange based on the judgement that a Bishop is *always* more valuable than a Knight regardless of the position. Use this irrationality to your advantage and bate your opponent!
Secondly, a Bishop on g3 or g6 is a strategic contribution to control of the important e4 and e5 central squares. If, however, the d4 and d5 squares are the more important central squares in a particular position, then the value of a Bishop on g6 or g3 or is questionable, and the possibility of moving it to f2 or f7 to attack d4 or d5 should be seriously considered. This point follows from a rudimentary examination of what a Bishop on g3 or g6 can do. On the other hand, there are also possibilities of coordination between a Bishop on g3/g6 and the Queen's Knight. If the Knight moves from, e.g. c3, to b5, then there is a double attack on c7, which is the amateur's way to threaten a fork on the opponent's rook and king. An opponent is often caught unaware of this threat, and will have played his Knight to, e.g. c6, and be unable to counter this naive attack. Still, even if your opponent pays attention, the *threat* alone can be enough to prompt a6 or Na6, and cause him to use time or diminish in central influence.
Defensively, I fear that the Bishop remains a poor defender on g3 or g6, except as an obstacle between a harassing rook and the g pawn.