http://www.superiorthreads.com/video...ster-part-one/
This may have been true for older poly threads, but thread like batting and other things related to quilting has improved immensely in recent years.
Many modern polys are weaker than cottons, per the video. There's also a part 2 on their site.
I use everything and anything on my LA. Poly is definitely the easier one to quilt with but not the only choice. A confident quilter should be able to use any thread you really want but don't be afraid of poly.