I heard it too - mealworms should be used as occasional treats, but not as main food source for birds. Bellow is link that explains it.

Here is a link that might be useful: Mealworms info

I cannot think of any reason why offering mealworms to wild birds would be harmful. When used as food for a captive bird, such as an injured or orphaned bird that does not have access to a variety of other food sources, they may be nutritionally deficient. Commercially available mealworms and crickets can be "gut-loaded" by feeding a nurtitional food source to them prior to offering them to the bird. The average wild bird that is using your meal worms as an occasional snack and even feeding its young with some of them is not going to be harmed. They will feed on many other insects that will balance the diet (and fruits and possibly seeds depending on the species).

I feed mealworms year round. They have never caused any harm. The average number of clutches in this area for Eastern Bluebirds are two per season. The Bluebirds here almost always have three clutches when they're supplemented. And as Myfrozenlittlepond pointed out, they do not rely exclusively on them. They still feed their young all sorts of things. It just makes their life a bit easier. They do not come to depend on them. If you run out, they simply hunt for insects.