I feel luna is very much relevant. It is all in how you use it.
I agree to the fact that it is one of the least desirable resources in the game... and that may be confusing to most since gold in games typically are the opposite of that. I feel like a jerk saying so, but I personally have used this negative bias towards luna from fanta players to my advantage in my trading.
I also do agree with the notion that it doesn't directly help in trading for rare cards or high tier items... but if you think outside of the box, I myself have found indirect routes to get what I need with luna all the time. It is just not the road most traveled.
Be creative: luna is vital to fanta's economy
Long live the luna!^^