There are examples of words being de facto defined on this site. Take a look at a question like How to translate "to pledge"?. The top answerer suggested to translate pledge into promespagi, even though the word had virtually no hits on google. Now, the first result when googling pledge esperanto is promespagi.
There are at least two problems with that
We enforce words that wouldn't become a part of Esperanto "naturally". This may very well increase the number of Esperanto words which makes the language harder to learn.
Defining new words is quite opinion-based. Besides all the normal problems with opinion-based questions, I will point out that opinion-based questions favor high reputation too much. Therefore, users with high reputation has too much power over which new words to define.
My solution would be to downvote or perhaps even report answers who are making up words. In stead we should answer with the best possible translation with already used words. If something does not have a good translation in use somewhere, we could tell the questioner. He/she could then invent an expression by themselves which would have to spread through its actual use - not through a SE-approved first hit on google.
At first, I had my doubts about this post, because it is good to have a place to make new words together. But now I would like that place not to be SE because of its structure and its authority.