Conferences without proceedings worthwhile for a PhD student?
I'm not familiar with your area, but I am strongly skeptical that it makes any sense to divide all conferences into two categories, those with proceedings and those without, and to declare based on that one binary bit of information that those in one category are more prestigious and more worth attending than those in the other category.

In other words, as with most things in life, the answer is it depends. Given two conferences you are considering choosing between, I would say look carefully at what topic each conference is about, how close is it to your interests, how good are the networking and learning opportunities you will get there, how attractive is the location, and yes, whether there is an opportunity to publish a proceedings paper; then make a decision.