Why, you ask? Sure it's fun to hop into some tight black leather, especially after wearing delicate robes all day long. But there's a more important reason.
Blizzard's outfit designs are more miss than hit. Here are some of the robes I've had the "pleasure" of wearing, and one that is inevitably in my future. I’ll neither explain nor defend my design-level assessment, so don’t try to haggle me on my numbers.
I find Umbral Brute to be particularly hideous. And many cloaks are just plain rotten, but hiding the cloak looks just plain weird. And the T9 shoulders? Halp!!
So to ninja I go, nearly every raid now, all the time. I just despise looking at my toon. And the ninja costume is really really good. So it no longer matters to me what sort of crap designs we get in the future. I've always got a good one to cover it up.
There was a solid stretch of designy goodness, when I was wearing mostly T8, and my priest looked badass in the best, most priesty way. But that was a blip on the design radar; mostly my guy looks like he got dressed by some overeager 19-year old artiste with no sense of restraint, color-matching, or style. Yes, I do really hate Umbral Brute. Looks fine on a lock, just not on a discipline priest.
It does give me great pleasure to choose the correct pet to go with my ninja, reflecting both my mood and the encounter. Ninja + errandboy, ninja + arcane elemental, ninja + Chuck, etc. He's an oddball ninja, my priest. But he's not gonna take off his leather for just any old dress.
Postscript: An ulterior motive
I like to heal pug heroics half-naked. Just for the challenge of it. Wearing a ninja suit covers up my prank, so unless I'm inspected, it will go unnoticed, and I’ll be the only one to know that I kept up a noob tank with so much skin exposed. The fact that I often sport Anathema with my leather, however, and that doesn't get mentioned, is more than a bit unnerving.