I don't think that anyone, with half a brain, has ever said speed is not valuable. The criticism isn't of speed, but as Plumpamania aluded too earlier of getting caught stealing. I don't know the exact numbers, but essentially the argument is that unless a runner is successful a certain percentage of the time, and it differs depending on game situation, they are hurting the team by attempting a stolen base. In the long run, if your team gets caught stealing 27% of the time, they will score less runs than if they never tried to steal those bases. Again, I don't know the exact numbers, but they are higher than most would expect, and most teams score less runs by trying to steal bases than they would if they attempted less steals.