One big downside to monthly/annual plans is the cumulative entanglement factor.
Many of the kinds of people ThoughtStreams is aimed at probably already have several tiny subscriptions. Personally I have annual Flickr Pro membership, several domain name registrations, a VPN service, and monthly subscriptions/fees for Libsyn (podcast hosting), a small public radio membership, Spotify, Feedbin, and whatever else slips my mind at the moment.
After awhile you get wary of adding another little mouth to feed.