Is it worth the money? Yes. Is it necessary? That will depend on a number of factors like speed of internet access, the quality of your router, and the amount of wireless congestion where you live. I get good speeds but my router is a little old and my apartment has a lot of congestion, and the wired adapter removed packet loss and lowered the amount of variance my Wii U was suffering. This in turn made it so I can have a smoother online experience, as long as I am fighting someone within my region (I live in the us, so I have to avoid Canadian players on Smashladder as the I just don't have the quality to play well).
If you want to see if you need the adapter, I'd recommend downloading a wireless interference testing software (one that uses your computer's wifi card to see just how much is being broadcasted into the air) and also doing a speed test with your Wii U to determine how much loss and variance you are experiencing.