3rd party batts are a hit-or-miss affair.
Even the "best-branded" 3rd party has got really suspect QC. I was one of the stubborn people who insisted that 3rd party batts represented better value for money, but from bitter experience, they're mostly rubbish.
Higher mAh rating, "cells made-in-Japan"... rubbish. It's especially nerve wrecking when you've got your main (original) batt supported by THREE 3rd party batts and the combined duration of the three was just a fraction of the original batt (because one of them couldn't hold charge).
So you might think that you've got a fistful of back-up batts, but what good does it do your confidence when you wonder if 3 out of the five you've brought may spontaneously fail?
You spend so much on the camera and lens already, and you still want to stinge on the single most important piece of accessory for the camera? How much can you save, really? $50? $100?