I guess it could help getting some speech practice, but the problem is that your conversational partners in your dreams will very likely make the same grammatical or vocabulary mistakes as you and your speaking won't improve. Speaking to others who know the language broadens your vocabulary and helps you correct other errors, but since talking to someone in your dream is technically like talking to yourself in a way, none of the people will know the language any better than you, so it stands to argue that you can't really get better. If you just want to practice speaking what you know fluently, then I guess any practice is as good as the next one so long as you speak, so it might help there.