Flannel as flats, thoughts?

I've read that it can be used as a flat and it's pretty absorbent. I've been trying to build my stash so I've bought some old flannel and some cotton sheets and receiving blankets. I plan on using mostly FST. The other day my grandma got her panties in a twist because "I shouldn't be using flannel." We argued for a minute, but she never told me why I shouldn't be. Is there a reason I'm missing? Do any of you use flannel?