I have not noticed a single problem with Sneaky's nail being removed, as far as balance, or anything else. She might have less gripping surface when she pulls but we have not competed or worked on WP since her problem, which was a rather uncommon fungal infection. Her nail was completely removed in November last year, when they did the biopsy, and the vet who did the procedure did a pretty good job.
It seems that some of Button's nail is still there so it should re grow. Tux did a number on one of his nails in November 2010, which is now fully grown:
and another on a toenail in the back, a couple of months ago, and it is growing back: