If I remember correctly, REI doesn't care how many 'memberships' an individual has. There's no need to enroll pets, trees, or even to make up names! REI is happy to collect those fifteen dollars upfront.
But Henry is right. Online outfitters frequently have sales. 20% is just nice, not spectacular. Add to that, REI prices tend to be higher than some.
In a way, store selection is like gear selection -- picking the right tools for the task at hand. For me, I shop at REI for big-ticket items on sale that I want to try but am not 100% sure. REI's warranty and service are pretty hard to beat. But for the rest of the stuff, I tend to just shop around for price.