Don't worry about the quilt police...they are more interested in those of us who should know better! It'll be alright, I'll keep 'em off your trail.
Seriously though, the only reason to worry about it not being 100% cotton is if you don't prewash your cottons, they may shrink while anything with poly in it won't. Other than that, it's all about how tight a weave you prefer for your fabric, and whether the broadcloth is the right texture for what you want to work with. If you're asking about sturdiness, it's perfectly fine.