Skype is a P2P application. The reason they can get such great performance out of necessarily busy activity like sending digitized audio around is that everyone who logs on helps each-other pass these packets around. While your node is relatively calm, it is used to transfer _other_ peoples data. Likewise, when you need to talk, the load is balanced to other under-used nodes.
This would be the reason for the "utilize your connection" verbiage in the EULA. Any P2P app (if honest) must also admit to this. It's how it works.
This is not automatically "spyware". It is the way P2P apps are designed. Please, people. Learn to research a little before freaking out.
Now, there are issues with how Skype encrypts traffic that *might* make it possible for an evil client to collect your conversations (or, more likely, part of your conversations, because the packets are spread throughout the network as load demand). This has yet to be proven, and Skype is being criticized for not being completely open in this regard.
Anyway, if you insist on using services like Skype or email for *secret* conversations, you are crazy. Everything you do on the internet is essentially written on a postcard unless you take the time to put it in some sort of envelope. In this case, there are some people who suspect that an evil hacker could open your Skype envelopes and re-seal them without you knowing. Serious (if true)? Yes, certainly. End of the world for most of us? Most assuredly not.
However, just because you are seeing network traffic on your node when you run Skype does not mean anything other than it is working according to specification. You can't get something for nothing, and if you want to use Skype the idea is that you don't mind sharing a little bandwidth so we can all use Skype.