I don't want anyone to know I'm carrying. I'm never sure if it's an obvious deterrent or makes me a bigger target. When I see someone with a gun on their belt, I can't keep but looking at them and making a mental note of their movements.
Also, it depends on the state, but some cops are very anti OC and I'd really hate to get harassed by some activist trying to make an example of me.
...to be fair, there are morons out there that intentionally bait cops to get a youtube video out of it.