There is nothing wrong with constructive criticism, and pointing out errors — even fairly minor ones — is important and useful. The difference, though, between people who want to find out something about the real world and people who just want to score political points, is what is made of those errors. That is the test of constructive scientific dialog. Specious accusations of fraud, plagiarism and the like don’t pass such a test; instead they simply poison the atmosphere to everyone’s loss.Peer-reviewed journals have well-defined rules about these things, and the best one is that have open and established procedures, and histories, and you can trust them. You can go look them up in the library if you want, even papers that are decades old.
Blog posts, on the other hand, can be modified without notice, or disappear, or make accusations outside the scientific process that get linked around the world and which are too often inflammatory, like McIntyre's smug "I gotcha again" posting of the definition of plagarism, when, as Realclimate explains, it certainly wasn't.
That's a big part of the climate change scene nowadays -- it's about scoring political points instead of "understanding the real world." It reduces it to a level the public can understand, but unfortunately it reduces it to a level the public can understand.