If the pool is stealing from miners, it's not like sometimes they're going to cheat and sometimes they're going to sneak in bonuses.
I think this is an excellent point. Pretend you are a pool operator out to cheat. Having 5 visibly bad days seems like a bad way to steal. It would be both easier and less visible to skim off a small amount on a constant basis rather than inserting code that robs a large amount on a set schedule.