We are members of BoondockersWelcome, a part of Harvest Hosts. There is no obligation to purchase anything from a farm/winery etc with BoondockersWelcome. I am not sure if a FWC would be accepted on Harvest Hosts as the grey water storage is external. I have not been able to find out if that is allowed or not. Harvest Hosts membership is also more expensive than BoondockersWelcome.
We have only stayed at a host site a few times but have hosted at our home 30 times so far. It is very relaxed, we meet interesting people, learn about other's adventures, and get to act as a tour source for our community. We love hosting!
Definitely look into BoondockersWelcome. Often you are staying in a driveway or along a residential street. There are times when there aren't wild camping options and we don't want to sleep at a rest stop.