Is Math GRE not worth it?
The vast majority of math Ph.D. programs in the US will require both the general and math subject GREs, so unless you specifically avoid those schools specifically, you probably will have to take them.  How schools use the GRE varies a bit, but for someone whose record isn't as strong, it can play a valuable role in reassuring schools, since it's the only thing they have which is uniform across schools and recommenders.

For schools that don't require them, they will likely assume that you have taken the subject GRE, and will assume that not sending in your score means it is really bad.  If your score is really bad (less than 30th percentile), then it's probably better to not send it in, but if it's not abysmal, then probably you should send in the scores.