I used to be a critic of sliding, but I've even found myself doing it lately when I'm scrambling for some shots so I've changed my mind.
As long as sliding isn't done for style, it can be a legitimate way to slow down and change directions, and probably is a natural method of protecting the ankles. If you watch Djokovic or Monfils slide, you'll notice that they spread the legs out wide and drop their center of balance. By unweighting the feet some, it is easier to slide, but it also protects the ankles if you lock them. Sliding is a controlled way to slow yourself when you need to stop quickly but want to protect yourself from an ankle roll-over.