I read some information about this recently. It said that Serger thread was 2 ply and regular quilting thread was 3, but it also said Serger thread was okay to quilt with. I was glad to read this, as I have been using Serger thread (a gift) for several quilts, without realizing it was Serger thread. My sister finally pointed it out to me. That's when I researched it.
Anyway, I intend to continue using it without any concern. Hope this is helpful, though I will read this thread to see what else I can learn.
Dina