A PM is just a tool that helps you get the job done more efficiently. Do you need one? No. But if you want to optimize your time spent on the bike, it's definitely the way to go.
I know some really strong guys who ride without any computers, some guys who have gotten really strong by using their PM's, and guys who are ditching their PM's because they've used them to develop a very accurate feel of RPE. I've been using a PM for about two months - my experience is that it has helped me to understand various efforts and where my strengths are so that I can race to them.