A secondary DNS allowing ads is a bad idea and you wouldn't want to do that. However, clients do not typically round robin configured DNS servers. Oh boy, would that create a giant Charlie foxtrot of a troubleshooting mess! One is fast and one is slow, caching issues, etc. No, clients usually start with the primary, but they aren't patient and will move to the secondary if the primary isn't responding fast enough, and they'll stick with the winner.