I have no reason to prefer it anymore other than if it’s the best on route and price. After all, it’s undifferentiated now.
Thing is, buying miles is normally a really poor use of your money, because the redemption rate isn’t great, and airlines devalue miles all the time. For example, the lowest option at delta is to buy 2000 miles for $70. That’s 3.5 cents per mile, but you can only expect to get a value of 1.25 cents per mile when you redeem them. Which only comes out to $25 in value, loosing you $45 — and that’s assuming you wait to spend miles for a good deal. (Redemption rate is worse during more popular flights.)
Airline miles are just not worth much, which is why people chase like hundreds of thousands of miles at a time through credit card sign up bonuses.