There is one major factor no one ever brought up. You purchase pre overclocked hardware it does not void the warranty. You purchase a vanilla card and over clock it yourself, you void the warranty. Also, even though you are correct with things like firmware flashes and riva tuner, it is very easy to over clock your video card. However, I have seen a lot of video cards go bad a lot quicker because it puts more stress on them.
Cards which are designed to be over clocked tend to have better electronic components on them. My friend had a vanilla 6800 that he over clocked to just under a 6800 GT because they are physically the same card, but after about 1.5 years of use it dies on him at times and gives artifacting. So, its a risk you have to take.
which kind of all goes back to my philosophy is that over clocking is dumb and really is not needed.