Well, actually it is not evil, it is tricky (I'm just being a populist). If you are not a computer geek I will try to explain you why it is so.

First, it hides its network traffic: the ports are randomly generated and the network packets are encrypted without an obvious signature. Someone might consider it as a feature, because your network administrators have a little chance to know that you are running skype or to block your skype connections. link (in Russian)

Second, skype is peer-to-peer and it uses many different protocols (not just TCP). This means that any active skype client can be used as a gateway to the network. For instance, if you don't have the internet access, but you are in a local network with internet access and someone else is running skype, you will be able to use your skype as well. It might sound again like a feature, if you ignore some viruses that use skype network. link (in Latvian)

Third, skype client is very well protected against cracking and disassembling. It is not easy to track what kind of information skype is reading from your computer and what kind of information it is sending to the network. link (in Russian)

Fourth, skype consumes more system resources than other messengers. By this I encourage most of my friends to switch back to icq/msn/yahoo.