I don't see friends-with-benefits as being "not-quite poly". I think they are fully part of poly life if you want them to be.
I say I'm poly because I want love with more than one person.
I love my boyfriends. I love my friends. And a friend-with-benefits is someone who, in my view, is somewhere in-between a friend and a boyfriend.
Therefore, I can also love a friend-with-benefits. (Notice I say love but not be IN love. There is a difference for sure.)
Ergo, if being poly is loving more than one person, the FWB can count.
Just my thoughts on the matter.