I always question buying refurb lenses. A LOT of photographers return new lenses if they are lower ranking copies in terms of copy variation, and those are the lenses that get sold on the refurbished market, already perfectly meeting Canon's quality requirements. They are perfectly acceptable in terms of Canon's quality control, just not the best copies you can possibly get, so a Canon tech isn't going to do anything with it except mark it as sale ready. For that reason, I buy new and exchange the copies that rank lower on the copy variation scale after I test them, just like everyone else that buys new. If I bought refurbished, I would never get above average copies of equipment, which is what I demand for my work.
If you love refurbished, that's cool and you can defend it, but I have tested my equipment with refurbished equipment held by friends several times to experiment. My equipment has always outperformed theirs. They do save money over me, however.
I see where you are going with this and agree to a certain extent. However there are manufacturing tolerances in the body too. That same plus tolerance in the lens when combined with another plus tolerance in your body will result in crap for you. However if I have a minus tolerance in my body, that lens may work brilliantly for me.
Furthermore I still have the ability to return a poor refurbed lens and have in the past. Just because I bought a refurb doesn't mean I would lower my standards or expectations of that lens over one I bought as new.

