Yesterday, I was kind of relieved that the museum isn't free. As a member, I go there very often to appreciate history but also to sit in a chair and work on my laptop with the richness of history surrounding me and inspiring me.
Yesterday, as I sat in the old wing, several people were sleeping on the couches with their shoes off and snoring... it felt very cheap, very low class. Others were sitting in galleries speaking very loud, telling jokes... it was disconcerting. I couldn't appreciate anything at the museum.
Now imagine if just anybody could walk in there. We got a taste of that during the opening of the Crystal. People who know nothing about the culture and history within the museum went there and behaved obnoxiously, running up walls, shouting, chasing after eachother, uncovering pieces that were veiled because they weren't ready for display.
If you don't want to pay $20 bucks to visit the museum, the odds are you're not that interested in it to begin with and likely don't know the proper etiquette to follow in such a venue.
For those who can't afford it, like Urban Shocker said, there is a Free all day Friday once a month and Free after 6:30 as well.