I think people should learn whatever language they're interested in, be it Esperanto or Sanskrit. What baffles me is that, even though most constructed languages have their main point of interest being simply the fact that they're artificially constructed, Esperanto is still an attempt to create a lingua franca
. That in itself sounds like an attempt to exchange culture for commercial facility (or "world peace").
I believe anyone who has seen Guarani being endangered by either Spanish or Portuguese, or Hawaii's struggles to preserve their own language, would feel slightly troubled over an artificial language. Preserving their language is a means to preserve their culture, and those artificial languages preserve no culture at all. Creating a language, in the eyes of the endangered one, is fabricating more threats.
And I don't mean that it is absolutely
a waste of time, but that it is, in comparison to learning any other language, a waste of time akin to that of learning Klingon. But again, a man should do whatever he's interested in ¯\_(ツ)_/¯