Business wise, an additional advantage of Skype is that you can set up a phone number in another country, which then rings on your computer.
For example: if you have clients in the US, you can get a US phone number and have people call there. That takes away a huge barrier for (potential) clients.
Skype is good, If you work all times of the day and night and don't mind being woken up by someone who wants you to do something. However, clients' minds work differently. They may not want to call another country/continent, they worry about what time it is and so on. Giving them a 'local' number to call, means that they are more inclined to call. So more calls mean more business and money.