I used to shift without the clutch all the time. I knew the RPM for any given speed in any given gear in my Accord and could rev match to shift, downshift, whatever. The problem was that I had to look at the tach every time I wanted to shift like that. Then I found you could just put a certain amount of pressure on the shifter and it would fall into the next gear without having to look at the tach, so I did that.

The problem with that is, if you don't get the pressure exactly right, it will grind the crap out of the gears as it partially meshes. I was driving my old DSM around without using the clutch because I was lazy, and being in that lazy mood, I didn't put enough pressure on the shift into third, ground the crap out of third, and screwed up my synchro such that it ground into third at high RPM from then on.

That made me realize that it was dumb doing something that has no real benefit and, if you screw up, can trash your tranny. Now I'll only do it if there's a good reason (blown slave/master).