I was riding with a friend who was on his Valk Tourer. He locked his rear cartar. The back of his bike started bouncing out of control. Scared the poo out of both of us. Next time I saw him the cartar was gone. LOL...
Another list I'm on had a big discussion over stopping distance changes with a passenger added. I did a bunch of tests with differant weights. (Interesting result - the added weight did not increase stopping distance much. Read in reverse, a lighter bike will not stop significantly faster - born out by MCNews' reviews.) This was pre-car tire. After the car tire, I repeated the tests and was able to shed about 5 feet from my 40-0 distance (44 feet to 39 feet on a slight downhill). I have ABS so the trick is to learn how much more presure you can put to the rear tire.
The better news if you don't have ABS is the tire is much less likely to fishtail when it does lock up.
Keep in mind, stopping distance is a function involving the square of the speed so doubling your speed quadruples your stopping distance which my testing confirmed. At 80, I should shave 20 feet off a panic stop. I should retry my stops now that I have more practice.