That may not matter. If you need a last-minute flight that costs more than $250 each way, which is almost any flight, you can just buy the necessary points to create a standard award.
Buying points? That sounds risky. Just because last minute flights can be available doesn't mean they will be. It's at best a 50/50 proposition and for some city pairings and times of the year when flights are at capacity, just forget it! I feel as though your tip should at least contain a disclaimer, since southwest doesn't allow us to check standard award availability unless we have awards to spend, and you can't unbuy points that were bought on a whim.