Agreed. I only "subscribe" when the lead-in revealed game is one I consider a good deal at $12. I then cancel if the next reveal is something I am uninterested in. I've been fence-hopping like that since the very first Humble Monthly, so not only is it allowed, but as @malkav11 says, it's just smart money if your library is already fairly large.
That said, they've had 3 lead-in games in a row (Dragon's Dogma, Warhammer : Vermintide and now XCOM2) that have enticed me to stay subscribed. The last few bundles have been some of the strongest since they started the program.