When I first read this, I thought it was a bit draconian. But the more I considered it, the better I liked it. I began paring down articles to 300 words, and in every case, they were improved by deliberately removing the unnecessary verbiage.
I've been a fan of terse writing for a long time. Presenting a thought with fewer words doesn't have to mean sacrificing rich narrative. Elmore Leonard, one of the top storytellers of our time, wrote in Elmore Leonard's 10 Rules of Writing "Try to leave out the parts that readers tend to skip."
This is excellent advice, not just for writers of fiction, but for journalists as well. Roy Peter Clark, long time journalist and teacher of writing, wrote a wonderful book entitled How to Write Short: Word Craft for Fast Times which discusses brevity at length (pun only partially intended). Clark explains not only why and how to use as few words as possible, but the circumstances under which writing should not be pared down.
Conciseness is a useful skill in all forms of writing, from fiction to technical documentation. In my opinion the AP's new guidelines are an excellent contibution to improving the craft of writing.