Oh, boy! I can't resist these psychological topics!
If what I write is too long, I'll leave a TL;DR at the end.
Here we go:
Having inner monologues might not sound that healthy or reasonable, but if you hear me out, you'll understand why this may be a productive practice:
first: putting thoughts in words is extremely valuable since sometimes we don't get what we're feeling. Yeah, we may feel a mix of emotions, however, we don't get exactly why. We're so concerned about the emotion explosion in itself than asking why we're having a hard time dealing with it. Deconstructing the thoughts in words makes you realize your feelings' motivation, which usually gets clouded by your nerves.
second: having a second voice to debate with is a blessing even if you're alone. This allows you to face contradictory arguments within your line of thought. Once again, as soon as you start monologuing you ask you some questions about a specific concern, and once that question is made, you direct your brain on thinking about answers to that dilemma you may be facing. Even silence might enough to you back in your tracks.
and third (I'll end with this one): this makes you constantly exercise your brain. Everything you think about is making you evolve your rational thinking. Of course, you can imagine silly things here and there, but when you encounter something you can't get over with, this is the point where you start thinking about resolutions. This way, you'll ponder about hypotheses and outcomes, which tends to orchestrate your view about what you should do over what you should not.
TL;DR: Monologues are healthy if done right. You may direct your brain in better ways than you think by putting things in words as you may find solutions that were once clouded in your head.
Still, dialogues are very valuable as well.
To digest something complex you must first deconstruct the idea in simples topics.
That's where our reason is born anyway: within the principle of the doubt.
Who are we to judge badly something so important and productive such as the monologue?