"You are a good person."
Translation:Olet hyvä ihminen.
Hei, hyvä kysymys.
Well, I think you do get a rather good sense of all kinds of constructions, and much of the vocabulary you learn should be usable in spoken Finnish as well, but the truth is that there can be quite a big difference between someone's spoken and written Finnish, it varies from person to person, and there's of course regional variation as well. So it depends. It would be quite difficult to teach spoken Finnish like this, because you'd have to choose and make compromises anyway. Which region would you pick? What kinds of forms would you allow? It easily becomes a slippery slope. But as a base I think Duolingo works fine, although as this course is still in beta, there's of course some room for improvement too.
Both a personal/style choice and formal vs. informal usage. Although it's actually the formal/standard written language where it's the most common to drop the pronouns (and they can all be dropped I think, but the 3rd person ones rarely are). In more informal language it's more common to keep them, not less as you might expect.