The observations in here are pretty accurate. JWs and the carts mostly stand out to people involved/previously involved with JWs. Thats why they stand out to me whenever I happen to see them. I will have a quick glance at the people by the cart, but then keep on moving. For the JWs themselves, I think the cart allows them to put in their time (as someone above mentioned) without having to be overt about trying to talk to people. It seems many JWs are just happy to stand by the cart and let time pass. Rarely do I ever see anyone actually stop at a cart. I remember how I kinda thought the carts were somewhat neat when I first saw them.
People are just too caught up in their own lives to take the time to stop and look at the carts or JWs, not to mention that most are not interested in the message.
Still, JWs get pretty creative about things sometimes. Not too long ago, there was an awake left in the waiting lobby at my mechanic. I wonder where else they get left.