Last summer was my first trip. It was during this time frame (Aug 17 - Aug 25th 2017). I think the answer to your question depends on what you mean by NEED. Will you be able to experience all of the rides at least once during your stay without UExp? Yes. Will you be waiting in lines greater than 30 minutes in the afternoons if you don't have UExp -- yes, definitely. Greater than 60 minutes? Yes, if you want to try the HP rides or Kong in the afternoon, expect a long wait.
I stayed at HRH. I thought that it worth the extra money to have UExp as well as the short walk to the park. I'm planning on the same this summer.
As for Volcano Bay, I really wish the UExp included this park.