I've been stretching relaxers for 12weeks since last August. I figure the less often I relax, the less often I subject my hair to chemicals. Even though the relaxer is applied to new growth, there's still overlap regardless - whether its by accident or from the runoff when you're rinsing it out.
ETA: I guess I didn't really answer the question. For me its about retention. Your hair is going to grow no matter how often you stretch.