The Banjo bot is just there as a bonus, but doesn't really provide value. The primary source of FREE upvote rewards is from the minnowsupport bot. We use Banjo to assist in tracking the use of the bots, and as a connection to Steemit for stats and other bits of information.
This is why we don't have a 12 hour timer restricting the use of Banjo on upvotes, since it won't provide any reward to use it more often. We only restrict the minnowsupport bot to prevent abuse and the draining of it's voting power too quickly.