Not a whole lot new in this article, though its "experimental" results confirm well-known phenomena regarding camera lens performance. In particular, small front-element blemishes do not in general significantly degrade optical performance.
That said, dirt or scratches become more important when there is greater depth of field, for example, shooting with ultra-wide angle lenses stopped down to small aperture. Naturally, how much image quality degradation is acceptable depends on the circumstances, there's no single rule that can be applied.
I wouldn't try to get into the laws of physics/optics governing these considerations, there are plenty of sites on the web for that. However I recommend taking a look at the LensRentals site for its wonderfully practical, entertaining lens information (including info re: user-level lens testing): http://www.lensrentals.com
That said, dirt or scratches become more important when there is greater depth of field, for example, shooting with ultra-wide angle lenses stopped down to small aperture. Naturally, how much image quality degradation is acceptable depends on the circumstances, there's no single rule that can be applied.
I wouldn't try to get into the laws of physics/optics governing these considerations, there are plenty of sites on the web for that. However I recommend taking a look at the LensRentals site for its wonderfully practical, entertaining lens information (including info re: user-level lens testing): http://www.lensrentals.com