Scubadetector who used to post here very frequently would tell you to get one and use it! He's lost a few aquaintances due to drowning in their waders.
That being said, I don't use one and probably never will. Like you, I've been in waders a lot during my life, trapping, fishing, smelt seining etc. Damn near died falling through the ice on a river in them once, could not pull myself up on the ice and the current was pulling me under the ice once the waders filled up. I took a deep breath, let go of the ice and let the current take me downstream a little where I knew it was shallow enough for me to get my feet on the bottom and push myself up through. It was only about a 20' float under the ice, and I was not scared as I remember, just mad that I was now soaked and it was a long hike back to the truck. I don't think a PFD would have helped in that situation, except for an easy body recovery. I think it would depend upon the situation, If I was wading in a fast river salmon fishing in AK, with the current constantly changing a gravel bottom and the water was very cold, I may be inclined to put one on. When a guy hooks a big salmon, things get tricky in deep fast water...The worst thing I deal with is waves coming in over the top, but I can handle that. If you think you are in a treacherous area it would probably be a good idea.
Mud