Putting these two ideas together:
I can think of four or five people I'm connected to on Facebook that are constantly at MPP/Wolf Trap/etc.
One album, 2 catchy songs and you're playing an ampitheatre.
I also know a lot of people who go to the MPP/Wolf Trap/Pier Six shows. And it does take only a couple of hits for a lot of those people-- they know and like a couple of the songs, and it's just a nice chance to sit outside and enjoy a nice night with friends, with a band playing in the background.
This is not that dissimilar from what happens at smaller venues either-- We've all been to shows where people just talk the whole time, even through the headliner. (early shows at the U St Music Hall seem to be particularly bad for this, and people have mentioned IOTA on this board as well). It ends up just being happy hour with a band playing in the background, which makes me wonder if people are actually there to see that specific band, or if they're just looking for a place to hang out with friends, and if they hear a couple of songs they know, then that's cool too.