There are various ways a person can respond the the discovery that I create languages for fun. The most common is noncommittal and polite puzzlement. A few people will be enthusiastic about the idea, especially if they're fans of the recent big films and TV shows involving invented languages in some way. Every once in a while, especially online, someone will object on the grounds that people involved with invented languages should, instead, be Doing Something about dying languages. This objection is so badly thought out that I'm genuinely surprised at its popularity.
First and foremost, anyone complaining about people messing around with invented languages has failed, in a fairly comprehensive way, to understand the concept of a hobby. Time I spend working with an invented language is not taken from documenting dying languages or some other improving activity, it is taken from time I spend with my banjo, reading a novel or watching TV.
Second, while it is true I, along with most language creators, know more about linguistics than the average Man on the Street, documenting undocumented languages is a special skill taking training I certainly don't have. In fact, most people with Ph.D.'s in linguistics won't even have such training. Do people going on about dying languages really imagine anyone can go out and do this sort of work? If someone has a nice garden near their house, we don't harass them about how they should be growing crops to feed the hungry, nor do we demand every weekend golfer go pro. What is it about invented languages that brings out this pious impulse to scold people for not doing something productive with their time when so many other hobbies get no comment at all?
If we step back to more modest goals than documenting a dying language, we're in much the same boat. There is little point to me going out and learning, say, Kavalan (24 speakers left as of 2000) unless I go to Taiwan and spend most of my time among the people who speak it. Sitting at home in Wisconsin learning Kavalan does nothing to preserve it in any meaningful way. You just can't really learn a language from a book. You have to spend time with native speakers.
Using other people's cultures — or fantasies about their culture — as a rhetorical foil has a long history. When Europeans were less approving of sex, they complained that Muslims were libertines, while others used this an example of a more sensible cultural trait. This is all part of the usual Noble Savage industry. The death of so many languages is a real issue, representing the permanent loss of a wealth of cultural and environmental knowledge. It deserves to be treated with more respect than to be used merely as a rhetorical club to browbeat people who have a hobby you don't like.