I requested a pat-down two days ago coming in through Detroit while transferring and they said "sure!" and were pleasant about it. I was expecting a battle. Also, it's not that invasive a process. I think we hear horror stories about it but I think more people should opt for it.
This is not to say that the whole process isn't stupid and broken. Certainly I was separated from my belongings on the belt which I didn't like.
How invasive the pat-down is depends entirely on how invasive the administering agent wants it to be. Some agents are actually rational logical humans who have already figured out that the screenee is not a terrorist, and pat lightly and send them on their way. Some agents are in the job for the power trip and will roughly squeeze every body part they can reach. And of course there's a spectrum between the extremes. It's impossible to know ahead of time what agents you will run into, and unsuitable to generalize from one or a few personal experiences to TSA agents as a whole. It's a crapshoot.
And of course the system is designed to be impervious to feedback. We can count on one hand, possibly one fist, the number of agents who have been disciplined for aggressive behavior. The only chance is to somehow raise enough of a stink to catch media attention. Any complaints officially made to the agency summarily disappear down the TSA memory hole.
This is not to say that the whole process isn't stupid and broken. Certainly I was separated from my belongings on the belt which I didn't like.