Yes, I agree, the premise that Netflix wants you to return the DVDs as quickly as possible so that they can send it to someone else is wrong. At any given time, you're going to have N DVDs out, and Netflix doesn't have any reason to particularly care whether they're the same N DVDs each day or N different DVDs.

When I return two DVDs to Netflix on the same day, I usually send them in one envelope in the hopes that the mailing costs will be slightly lower. (I figure that if their costs are lower that helps keep the subscription price down.) I've wondered why Netflix doesn't try to combine the discs that they send, but perhaps the cost savings aren't enough to be worth the extra handling that would be required on their end.