Since I am doing this I had probably one round where I got all. I think it is unfair to sign in and then not sending the cards. Sometimes there are good reasons, sometimes not.
BUT!!! In every single round there were some postcards where I said "Yes! Sending those 50 postcards was worth getting this single card." And browsing through my pile of hundreds of postcards makes me forget about those unfulfilled commitments and keeps me organizing this lightly.