This is a good point, but what you're actually doing with exercises like this is training muscle memory. It doesn't matter if you're learning Dvorak, QWERTY or how to chisel letters into a block of marble.

The advent of computers also killed one of Dvorak's selling points, which was a lower error rate at speed. Errors on a typewriter were painful to correct and took up a lot of time, but by the 70s and 80s, even typewriters were capable of reasonably easy correction. By the time word processors kicked in, mistakes were nothing to deal with.