For the vast vast majority of people, if learning a language is "immersion or bust," their entirely rational response will be bust.

That said, relatively casual language learning isn't likely to yield a lot outside of essentially entertainment value. I had four years of high school French which likely tops the relatively casual bar. On the one hand, I classify my French as pretty terrible. On the other hand, I've traveled to Paris with a friend who doesn't know any French and it makes me realize I actually do have a bit of working knowledge especially when it comes to reading.