At the master and GM level, of course it isn't. At the club level, I've noticed some respectable players, not masters but competent duffers, who never seem to castle. They also play the weirdest openings but that is another thread. Is there some plan behind it or do they just not get around to it?
us “competent duffers” prefer not to castle, to entice the opponent to mount a duffer attack on our exposed kings, leaving his own king easy to checkmate
In some situations, castling can be a blunder. For example:
If you know what you're doing, you should know why this is a blunder and how to punish it. Don't be like White in this game.