I definitely think that earning RR rewards with car rentals is worth it, even with the frequent flyer surcharge. Compared to other airlines, Southwest is very generous in awarding at least 0.5 credit (and, very often, especially with Hertz or Alamo, 1.5-2 credits) for virtually any car rentral. I credit all my car rentals to RR.

I have had good luck getting the extra credits with appropriate promo codes with Hertz and Alamo.