So, now that I've read the comments on a few of these open-access type articles, it seems like there is so much variation among academic disciplines that broad statements about "scientists" are untrue in many cases and probably isn't a helpful way to phase things. This statement is not true in every field: in Economics (my field, you can decide whether or not we are "scientists," but we certainly try to publish in peer-reviewed journals) there are many blogs by extremely accomplished and prominent Economists. They get a lot of informal but meaningful credit for it, in some cases much more than another top publication would bring (when you go from zero to one or one to two top-tier articles, it affects your career. When you go from 31 to 32, not so much).
Just for specifics: Jim Hamilton at UCSD (my alma mater) started and contributes to the blog http://www.econbrowser.com. His graduate students started landing order-of-magnitude better jobs and interviews once the blog was established (and, just to make clear that it's not sour grapes on my part since most of them are my friends: these are jobs that the students are quite well qualified for; I think previous students if anything got too little exposure). It's a great blog that's really well regarded among Economists, and everyone I've asked thinks that the blog is bringing him a lot of (IMO deserved) attention.
> This de-prioritization of peer review slows down the whole scientific communication system.
In any field that I'm familiar with, scientific communication does not happen through journal articles. It happens through conference presentations, seminar presentations, informal discussion at conferences and when visiting other research institutions, working paper dissemination, and (somewhat, but growing) online.
A lot of that activity becomes formalized by comprehensive letters of recommendation that are solicited for every major promotion (in academia, obviously), so it gets heavily rewarded and is taken seriously.
Peer review exists as a check on those steps to make sure the impressions are right. Polish up the arguments, have an editor and referees sign off on it, and then put down a copy for the record for the people who need it to look over carefully.
If there are specific fields where that is not true, please spell them out.