Is this absolutely needed for PE parts?

In short, yes. I've tried enamel, lacquer and acrylic paints on metal. They flake off, if no primer is applied.

I prefer to prime the entire model, but if you do not want to do that, you can try brushing Gunze Mr. Metal Primer, which is a clear liquid in a 40 ml bottle, onto just the PE parts. I use it quite a bit on small white metal parts when I build car models.