I tend to think that some folks just want to get their grubby mitts on the biggest and baddest stuff possible. I know I am one of them, and I suspect that over 90% of folks who would be likely to read this are too.
It's why some people buy a fast car. Not because they can drive it fast, not because they race the thing, just because they can own one and that it's the meanest thing going.
I use a 185/160mm F/R Avid mech system. Do I really need that kinda braking? 99% of the time the answer is a definitive NO. But I do dip into that remaining 1% where I really do use up all the brake has to offer, and while a smaller 160mm front rotor would do the job and put up with the load without too much trouble, why push it that hard if I don't have to? By spending a few extra bucks, and dealing with the extra finickyness of the bigger rotor, I have more brake than I really need most of the time, and enough brake for when I decide to push a little harder.
Heck, I know the extra initial bite the larger front rotor has is largely responsible for throwing me onto the deck once, and that is a VERY rare occurrence. But I still keep it there and have no plan to remove it. I think it may have saved my bacon more than the one time it threw me, so I think I am playing the odds and making like a bandit.
As for a Hydro, if I were in a position to get anything I might desire, I don't think I would lose the mechs. After a little bit of attention, they are smooth and reliable, and I doubt that I could match them with a hydro of any brand to be perfectly honest. While I like the silky smoothness of a hydro, I don't mind at all that my Avids are a teensy bit less smooth, simply because I the point am moot. Would I exchange my smoother than average Avids for say, a sloppy (from the factory) hydro? Do I look that stoopid?
At the end of the day, if you are running out of brake with discs, you at least have the option of using larger discs. I know of only one guy who ran out of disc at 200mm, and he ended up running 29" wheels to gain clearance on the 250mm rotor he had made up. That guy is a serious nutcase, and no flyweight, so I could understand. But for someone who pedals, I think there is more than enough brake available in any disc system these days (save a few selected and outdated ones).
My grandpa always told me "anyone can go fast, but don't forget you have to stop sometime… Make sure your brakes are good."