I'll take the opposite view. I do mileage runs to accumulate miles, and use those miles to buy business or first class tickets on international routes (I've done 9 award tickets to Australia, with a handful to Europe and domestic U.S., in business or first). I will pay a few dollars extra, and don't mind it, seeing that I bring in over 10,000 miles on any cross country trip. Those 10,000 miles are conservatively worth $150 at 1.5 cents a mile.

Now, I can't ever recall paying more that $25-50 extra when the fares on the airline I have miles with was higher than a competitor, but I would not hesitate to pay that. Maybe you're finding a higher price spread because of your route and carriers available? I just don't see a $300 difference on routes I fly.

If the miles are worth it, it does make sense to stick with a certain carrier or alliance, even if it does mean paying slightly more. Those miles have value.