It really depends on the situation and price differential. I try to look at it as "would I be willing to pay that much as a stand-alone mileage run for those miles?" If yes, then it's worth it to buy a higher fare, because then I get the miles and don't have to spend a minute of additional time on it. But within NA, no, the amount of miles accrued based on distance is generally so small that it is almost never worth it. And with the uncertainty facing Aeroplan right now post-2020, I am not going out of my way to accrue miles I'm not sure I can burn before then.