Well, there's no such thing as always.
h3/h6 is a common move to throw in making the bishop either capture, or give up the diagonal that gives it the option of going back to the center.
Sometimes that's too weakening.
Sometimes it's only slightly weakening, so it's just a trade off. Maybe I should say most times it's like this. A little plus, and a little minus. Most moves in chess are like this.
According to Del Rosario: A First Book of Morphy, the famous chess commentator C.J.S. Purdy said "Always unpin!".
Yet in Logical Chess, Irving Chernev says (for example on pp. 16 and 21) that it is better to submit to the pin than make a move which breaches the safety of your castled king's position.
What do you guys think?