We calculate member weight on a weekly basis, and then divide by member's actual posting frequency. So a member is not actually required to post every day. In fact, a user should receive the roughly the same benefit per share if they post once per week versus posting once per day.
In fact, most members with only a few shares would be better off posting less frequently. If a member posts too frequently for their share level, and does not receive much support from other sources, their SBI votes might not exceed the dust threshold.
But I understand what you are saying about bidding on the available upvote weight. I still think there is a fundamental difference, since it's an ongoing round and we use the STEEM received to increase the voting power through leased delegation. That makes it so the round cannot be 'overbid' with an immediate fixed loss like it could be with a bid bot.
In an 'overbid' situation with SBI, it would just result in a slightly lower (though still profitable) ROI until we can recover the target SP levels.

We our working on enhancements to our system that will permit us to also upvote comments (only as a release valve for members not publishing posts.) So when that is complete, people can even receive your weekly weight just from interacting with people, rather than from posting 'too many threads for too few readers'.