I don't like using them only because even though it's something you earn in-game, I feel like I'm cheating out all the hard work to capture a rare Pokemon. I think the only way I know it'd be worth using is if I happened to find a Shiny legendary.
I basically do the same thing as you. In my Pokemon Emerald, Sapphire, Diamond and Black I never used it.Do you use your Master Ball? What have you used them on in the past?
I personally often have a habit of keeping the Master Ball, thinking it'll be useful for something else bigger at some stage, and end up never using it!