"An artist should not be allowed to draw so much as a radish without the constant habit of copying the old masters." - Degas.
Renoir also had great admiration for the master artists of previous generations and made copies of their work throughout his career.
"When we look at the work of the old masters, we have nothing to congratulate ourselves on. What marvelous craftsmen they were! They knew their job; that's the whole secret. Painting isn't dreaming; it's primary a manual drill and one has to be a good workman." - Renoir
Now certainly they weren't recommending that we copy someone's work and try to pass it off as our own, but I think the way art is taught in our time, it leaves out the benefits that can be gained from making studies like these artists did.
There seems to be two kinds of copying that great artists of the past recommended.
1. Copying to learn, as is described above, and
2. Copying to find your style. I think this is what Picasso meant when he said,
When we do try to copy works by others, we see something distinctively different in our own that keeps cropping up. It's like something is pulling you away. This could be a revelation of individual style. Picasso was saying, pay attention to those things and run with it.
"What does it mean for a painter to paint in the manner of So-and-So or to actually imitate someone else? What's wrong with that? On the contrary, it's a good idea. You should constantly try to paint like someone else. But the thing is, you can't! You would like to. You try. But it turns out to be a botch...And at the very moment you make a botch of it that you're yourself."
-Picasso (Parmelin, Picasso: The Artist and His Model, and other Recent Works,
1965, p. 43)