If the theories of David Hockney are true then one could look at this discovery in a number of ways. As an aspiring artist one might be relieved that the old masters had some sort of aid. At the same time it may diminish the awe of these masterpieces to say that they were basically traced.
In my personal opinion it does take something away from the talent of the artist, but at the same time I am a bit relieved. Many of the works mentioned have been the standard of art throughout history. I've always tried to achieve that level of perfection without any kind of assistance. Even though there was perhaps an aid the skill and the eye were necessary to produce art at that level.
It did however help push art forward. With these lenses artists were better able to find the properties of perspective and other optical occurrences that help make the art more realistic.
While some artists may have used this technique I'm sure that not all have. The Sistine chapel most likely was not done that way due to the size and the fact that it is on a ceiling. Also sculptures could not have been done that way.
It's very likely that many works of art were done that way, but not all of them were. Even if they had used a lens to trace their image some people couldn't even do that. The painters of that time period still have my respect.