The point of promos (from the companies' perspective) it to gain new customers or increase business. It's not to give away miles left and right for stuff you would do anyway.

Now, getting miles left and right for stuff you would do anyway might be your goal, but keep in mind it's neither the airline's goal nor the car rental company's goal.

If you want the most bonuses, you have to be able to "follow the bonuses". I collect with UA/CO, DL, AA, and BA, and at different times I steer my rentals (mostly with Avis) to one or another of them, depending on who's giving the best bonus for the rental length I need that time.

If you only collect with one airline, you have to be willing to switch rental car companies, to wherever the bonus is. If you only rent with one rental car company, you have to be willing to switch airlines, to wherever the bonus is. Otherwise you'll never get as much in bonuses as the person who's willing to do one or the other. And no one will get more bonuses than the person who's willing to do both (choose whichever airline and whichever car rental company, from among several of each at least, that has the best bonus of all the combinations).

Meanwhile, many companies take a "breather" after a giant round of bonuses. Delta probably used up all the miles for rental bonuses they had alloted for a couple years with the 5K per few-day-rental with Avis they had for many months at the end of 2009. I remember I was between cars at that point, and ended up doing back-to-back-to-back rentals that grew my Delta acount close to 50k. (And, btw, I did that, despite Delta being the airline I care about the least of the several that I collect with! There was no way I was going to earn even half of that on those several months of rentals I needed, so I choose to earn toward Delta then, even though I've never done another rental toward Detla since that promo sended.)