I've been trying to figure out what the strategy is here. I suppose that if you don't want to add extra balances to your credit card, Paypal is an attractive option. And, you know, not everyone uses plastic.
What I suspect is really at play here is a mentality on Skype's part that the more purchase options they offer for outbound SkypeOut and inbound SkypeIn minutes, the more subscribers they will get. Can't really see a flaw in that reasoning.