I don't think they have to be mutually exclusive. Since stuttering has strong genetic links and it's safe to assume that there is something different in how stutterers process linguistic rhythm, it makes perfect sense that our brains would use the only fluency we're familiar with to form the basis of dream speech. To employ a bad metaphor, it's just as much habit as someone with a broken arm choosing to favor their non-injured arm on instinct. It is a habit, but that doesn't make the broken arm any less broken, you know? I do notice a lot of stutterers (and I've been one) have this nagging concern that maybe their metaphorical arm is no longer broken, but their instinct to not use it makes it impossible to tell. There's really no telling, but I'm personally a subscriber to the belief that if fluency were possible for me, the habit of stuttering could be ultimately overcome. Yet I can't overcome it. Instinct or not, it doesn't matter when that's the case. The end result is the same.