Security of credit
At least a pre-paid system would prevent what happened to me. I got an email saying a topup to my Skype account had failed - I use a prepay credit card for online things like that and I keep the balance low. My topup failed because there was insufficient credit. A topup was attempted because my account was hacked and all my credit was stolen. despite only using the official Skype client and a secure password, Skype said it must be my fault that my account was hacked and refused to refund me. I have up to date AV protection, I have a PC firewall and a gateway firewall (linux based) on my internet connection, and I've worked in IT long enough to not be clicking stupid links in emails.
I have not used Skype since and never will again...