Skepticism is healthy. I say this because I am a skeptic, but mostly because it is always... ALWAYS a good idea to question information. No matter the source, a piece of information is likely colored or skewed by the opinion of the author or presenter. Skepticism is a means of discovering truth or falsity.
Can a skeptic be skeptical about real evidence? Sure, but those aren't true skeptics. True skeptics are leery about non-evidence-based theory. Those who dispute real evidence are... well, they're wackos. Example: the moon landings were faked. It's a preposterous notion when you consider factual evidence.
The conspiracy theorists who perpetuate the idea that 9/11 was staged by our government for it's own gain are just such wackos. To these people, out-of-context quotes, shadowy pictures and black helicopters ARE their evidence. So Popular Mechanics decided to put some experts to work to "test" the facts and evidence of 9/11. I thought the article was fascinating - read for yourself.
Unless you're a wacko.
9/11: Debunking The Myths