How would a lightweight client keep track of which delegate signatures are legitimate?
Wait, they could just check if a majority of known-legitimate delegates accept a block produced by a new delegate.
But if the delegate turnover rate ever exceeds fifty percent in one round, then the lightweight clients would be unable to pass that point, and in-fact the kicked-out delegates would be able to form their own fork.
But wait a minute, a fifty-one delegate bloc could make a fork that even heavyweight clients would accept. I guess this objection is irrelevant because you always trust fifty-one delegates, heh.