Well I must admit that while I do love languages and Tolkien was a major influence on me wanting to study linguistics, I think in many ways I agree with your friend.
I love the vocabularies in the Silmarillion and HoME and trying to decipher the meanings of names, but given that there are so few substantial Elvish texts in the works, I am not motivated to learn elvish to converse with people.
However it is a harmless occupation - I have always wanted to learn Latin properly but keep feeling another modern language would be more useful. That said I use the language I studied for 12 years so rarely that utility may not be a justification. There are many seemingly pointless hobbies that people enjoy - if you feel it worth while to spend your time thus in order to communicate with like minded folk it is up to you. Possibly no more eccentric than Esperanto. Also you might pick up a working knowledge or the source languages for Quenya and Sindarin (Finnish and Welsh which might be marginally more useful)
However the only real caveat is that I did read (possibly an urban myth) that psychiatric hospitals are starting to have to employ speakers of Elvish and Klingon to communicate with patients who have taken the whole thing a little too far....
