Yes, they shouldn't be a regular part of their diet, because, like people, rats generally prefer junk food. So you'll end up feeding them your mixture, and they'll eat the seeds first, leaving behind the other food. And then it would only be natural for you to think that because they like the seeds so much, that they should get more. And then you'll end up with a bunch of fat, greasy rats with the libido of a rock....small litters, crushing babies because they're too lazy to actually nurse, so they just lay all their weight on them.