Yes, people should not rely entirely upon Penn & Teller for information. No problem there. P&T are entertainers. They do not pretend to be scientists. They are popularizers of skepticism. Personally, I don't have a problem with entertainers or popularizers. I like being entertained. If you're being led astray because you're getting all of your information from P&T, and not thinking critically or doing your own research, that's your problem...or perhaps the problem of the educational system.
The issue I have with Jenny McCarthy has nothing to do with her being popular and famous. It's with her message. Her message is harming people. P&T also have a message, and, overall, I think it does much more good than harm. I don't criticize McCarthy because she's "overstepping her competence". That's a separate issue. Indeed, I think that if you are passionate about something, and you are in the public eye, you should use your soapbox. No, the problem I have with her is what her cause is.