Tons of folks (think: grandmothers and parents and other potentially non-tech-savvy folks) have been using Skype to communicate with family members across the globe for many years. Simply expecting them to switch to Google Hangouts is a tall order.
Well, I'm not a big fan of Skype but I tend to use it for longer calls with family overseas (US<->Europe) because I experience quite significantly better video/audio quality. Few months back (it's fixed now I think) I even had freezing video every so often on Google Hangout.
Or screen sharing, it was completely unusable in Google Hangouts on Linux just 2 months back whereas Skype didn't have any issue and worked (surprisingly!) flawlessly.
I don't really perceive the network effect that much, everybody I have on Skype also has a Google account.
Because Skype always works. For example, most ISP in Russia still allow access to provider's network ever if internet connection is unpaid. Skype works, because somebody with internet who paid for it is gate for all unpaid users.
Skype traffic is almost impossible to block except some hacks about detecting his autoupdate.
Works usually yes, but that isn't all a good service needs. I'd like to enjoy using it rather than getting Skype-rage. Interface, inability to block calls (from iOS at least), I'd better not start or I won't stop.
Because I don't want to sign up for Google+. I dislike the idea of bundling their social data mining solution with just about anything, like a less obvious and impossible to opt-out version of whatever-toolbar bundled with software years ago.