Earlier quoted context omitted.
It depends heavily on the language course. Duolingo's Italian course actually manages to teach grammar and usage through their standard translation setup, while their Japanese course does nothing of the sort. It does not work very well for the latter, I've been through most of it by now and it's just translation-translation-translation, which doesn't teach you much at all, because the translation to English really do…
Are you sure you don't mean "gozaimasu" in the taxi driver example? I learned what little Japanese I do know from Duolingo and it was somewhat useful on a recent trip to Japan. Our tour guides were mostly impressed and certainly didn't point out any glaring issues with it (although that could be Japanese politeness).
And yes, in Japanese culture it would be extremely rude to point out errors in a guest's attempts to speak the language.