I see what you mean. With variable miles like UR or MR that makes perfect sense.
Ok, here are some numbers for you which are food for thought. The CSR $300 travel credit is triggered by any number of things including tolls. I happen to trigger that credit in tolls alone fairly quickly. Therefore, I look at it this way. I'm spending $300 in travel regardless because of the tolls. I need points that transfer to hotels or airlines but can also be used as cash through the portal. So it is a CSR or CSP. The difference between the two in AFs is $55 since I am spending over $300 in travel (tolls) no matter what I do. Let's say I want to purchase $1000 worth of travel through the UR portal. With the CSR it will cost me 66,666 UR points. With a CSP it will cost me 80,000. That is a 13,333 difference.
Keeping the CSP over the CSR now just cost me $200 in buying power because I wanted to save $55. I also had to spend a lot more just to earn those points when my earning is 2x over 3x. Hope that helps.
Granted, there are cards, if you are willing to juggle a little that can help you earn more UR points at a faster rate. Hope that helps