Well, it's not an issue of credibility. It's an issue of misplaced elitism that some Mac users display. Even some that actually have a functioning PC in their house. Hypocrisy is a very bad quality in a person.
Hypocrisy? Because I have a functioning PC and a functioning Mac in my house and I prefer the Mac that is hypocritical? That's odd. I've owned a Ford, a Chevy, and a Nissan . . . and I prefer the Nissan. That doesn't mean that I'm a hypocrite.
I didn't call you a hypocrite. You've not displayed any of the elitism I've been talking about. If I was calling you a hypocrite or an elitist, I'd just say it straight up.
Edit - To further explain, in case it's not clear, there are some people who will go around extolling the virtues of Mac's, how great they are, how stupid anyone is for using Windows, and so on. These people are elitists because they feel they are better than those bottom feeders who don't use a Mac. That goes far beyond simply preferring Macs to PC's. But that doesn't make a person a hypocrite. But those who go around being elitist like I described AND have a functioning PC in their house are hypocrites (as are most people who are elitist about anything).