Since I have a policy of never reading anything without attribution, I could read only the letters to the editor (ok they had bylines on those) so I only know about the article quality indirectly.
All of their letters to the editor used to be addressed simply to "SIR" as well, a practice they've only done away with just this year. Aside from a couple of pseudonymous regular features in each issue (Bagehot, Schumpeter) there are no bylines.
In 2013 they talked a bit about why they choose to operate this way:
http://www.economist.com/blogs/economist-explains/2013/09/ec...
[0]: http://www.economist.com/blogs/economist-explains/2013/09/ec...
It is The Economist. That's the whole point. There is a (good) reputation attached to it. If they fail to live up to their own standards, then that reputation will crumble and people will stop reading and quoting it. It's a brand.
We used to have brands for products, like RCA, Westinghouse, Philips, etc. But these have become just nametags that are bought and sold, and no longer mean much, or anything. But there are a few brands left that still mean something.