A queenside castle is a perfectly fine move, in the right circumstance. First and quite obviously, the a pawn is no longer defended and could simply be taken if it was attacked. Second, it does tend to be a more vulnerable position, as the king is more in the center and exposed.
But it also has it's advantages. If you happen to have all of the queenside pieces cleared off first, that could be a reason to castle that way. Another reason is that it puts the rook squarely on the d file, somewhere you'd probably want it. King side castling puts the rook on the f file, which is not usually as benificial, and therefore requires another move to activate. Also, castling queenside, if your opponent has already gone kingside can be quite an attacking move. By putting your king on the queenside, you allow yourself to attack with your kingside pawns, without worrying about your king's safety.
Originally posted by AkashicGame's still on the go, so no comments, but in this case I see castling Queen side as an important development move leading to an attack. Maybe it doesn't work, but it's still the right thing at move 10:
Trust me buffalo you're rating will skyrocket 400 points in 4 days if you follow those tips.
Game 1042490
The value of castling queenside depends on the position, period. Statistically, it is probably more dangerous in a greater number of positions, but if you are debating between long and short castling you shouldn't be thinking statistically. I would refer those who think that queenside castling is bad tactics, period, to games played in the Sicilian opening where white increases his/her winning chances by castling queenside. On the other hand, castling queenside in the queen's gambit declined is usually unpleasant for white.