It isn't always done this way. In fact, when experts discuss the newest and most exciting cosmological theories - dark matter, dark energy, inflation, string theory, etc., they rarely, if ever, present the pro's and con's. They present these theories as if they're done deals. Case in point, Neil deGrasse-Tyson, in the 5th installment of Seth MacFarlane's awesome Cosmos redux series on the Fox Network, describes dark matter in a way that suggests that it is a certainty - as if it exists as sure as the sands on you fave beach exist. Well, the truth is that we NEED it to exist, to keep current cosmological theory tied together. Now it may very well exist, but our theories could just as likely be flawed, or even ** shudder ** incorrect. It's not like there are beakers of the stuff in chemistry labs at MIT or anything.
Time to be thorough and honest here, kids. Especially you, NDT!