This issue is controversial, and I hope others will add to my response.
First, whether or not having a PMA certificate is a "requirement" depends on who you plan to work for. You may or may not find it a condition to being hired. If you are (or plan to be) an independent trainer, then see whether your competition boasts PMA certification.
According to the PMA's certification overview page, to take the exam you have to "Provide verification of Pilates teacher-training proficiency using as a basis, 450 hours of training on all apparatus and all levels of mat." So, that basically means you have to go through a standard comprehensive Pilates certification program to be eligible to take the exam.
... which of course begs the question as to whether the PMA certificate is really necessary. Unless and until the PMA manages to make obtaining its certificate legally mandatory for Pilates trainers, doing so is fully up to you.
Here's an objective article on certification basics.
Ultimately, your own training philosophy (classical, modern, eclectic, etc.) should determine which teacher training organization you choose.