I'm referring to cases where very few people living in your area speaks that lingua franca that you want to learn.
Depends on how serious you are about learning it.
For example, if you become fluent or close to fluent, it's a good skill to put on your resume and you'll be a good travel buddy if you go to where said language is spoken.
If you're just 'dipping' into it, you could potentially make friends with native speakers (/int/ could tell you about that stuff) and learn more.
Of course, it's really up to you and what you want. Do you want to put time and energy into it? Do you intend to use it? Where would you potentially use this skill?
I say if you have the time, go for it. Language learning can open up all sorts of cool opportunities.
No knowledge is ever perfectly isolated from your reality.
It might not be the most valuable thing you could do out of the infinite possibilities, but its not exactly a step backwards, is it?
You could also look at it as a risk-reward thing. Sure you could learn something everyone else is learning, for known benefits. Or you could make yourself unique, and hope there is a niche that no one else can fill.