Indeed, on cheap rates, only airlines that earn flat miles (rather than per-spend miles) are a good deal without a promo (or with a multiplier promo like 3x). Southwest is one of those (very few).

It's only when a flat (not multiplier) promo comes along that I choose to earn mlies from legacy airlines directly at Marriott. That last time that happened was early in 2012 when they were giving 2000 bonus miles per stay. So I was doing lots of cheap one-night stays, and who cared how few "base" miles I earned, since I was earning 2000 bonus miles per one-night stay.

But without a flat bonus, yes, earning miles with an airline where you earn per-dollar miles is not a particularly good deal (unless it's to trigger an airline-end promo such as US's Grand Slam).