Is Mack still an active PC? I can't remember a recent post from him and I'm too lazy to search.
Jabs at Cas aside, last time we were trying to be stealthy Robert got into a street fight with hundreds of people. Made it onto national television, and had otherwise blown their cover. He also tends to be loud, blunt and enjoys hitting things.
I could see why people would be a little less than comfortable with him on a 'stealth' mission...
And no, Skye's invisibility is much cooler.