Depends on how much it costs. If it's less than $5, I will probably buy at least a yard. If it is regular quilt-shop price, I will probably buy half a yard or a fat quarter, if I just love it but don't have any particular project in mind. I like scrappy, so it will be used, and I don't have a huge investment.
I sort of "collect" fabrics. I would like to buy more, but realistically, I am not going to be able to make a whole big quilt out of every fabric I love, and I have limited budget and storage space. I love charm packs for this reason.
Part of the reason I like scrappy quilts is because I can use and "display" a lot of different fabrics that I like, all at the same time.