The essay, you referenced, is "How to do philosophy". If you're going to do philosophy, the approach he proposes includes learning from past mistakes.
That said, I agree with his answer to the raq - you'll be better served by doing philosophy in other fields. Still, I think that's a big difference from saying those classics are a waste of time. In learning to abstract concepts, learning from previous struggles isn't a bad way to go. I am agnostic, however, on the best ways to learn how to abstract concepts.
I can't think of any I'd recommend. What I learned from trying to study philosophy is that the place to look is in other fields."
http://www.paulgraham.com/raq.html