For flights to Europe I think I generally agree that paying cash can make more sense. I just paid $1900 for BWI-MAD. That was thanks to the fare war between oneworld and SkyTeam last fall.
For trips to Asia I get the most out of my miles in F or C. Those 12+ hour flights are where I really appreciate flying up front and the price of those tickets is seldom cheap, notwithstanding occasional sales to certain destinations.
But it's always worthwhile to evaluate each journey on its own. If you're miles-rich and cash-poor then spending miles makes sense.