Call me a skeptic, but I use Gutermann all the time with no problem.
Personally, I think your LQS owner is trying to sell you some more expensive thread.
The last time I bought 40 wt Sulky, and had to take it off my machine because it kept breaking. It looked beautiful, but was too soft for my freemotion quilting.
Just use what works for you, and let the quilt police go bark up a tree. :P