I also have not used Skype for training clients. However, with that being said, I think that under the right set of circumstances (e.g. working with a long-standing client who you have confidence in knowing your exercise techniques etc.), it “could” work. Nowadays, the internet is being used for many things including things such as ‘tele-medicine.’ I don’t see why this would not work, we as trainers just need to be very selective who we use it with and be comfortable with the client and their level of skill and knowledge.