Seeing how quickly the Windows Marketplace has been growing, the number of apps and developers currently working with the WP7 platform must be staggering and this new rule would greatly benefit them. Especially new developers who don't have a whole catalog of apps to vouch for their credibility and need to rely on the “new apps” section to be discovered.
This should also introduce more variety to the Windows Marketplace, with more apps from different developers showing up at the same time. Read up more about the change on the official Windows Phone Developer blog.