i only buy fq's if i need a (little bit) of something for a quilt....when i made my sister in law's sandhill crane quilt i wanted all neutrals and did not have alot of variety...buying fq's allowed me to put more variety into the quilt than i would have been able to had i been buying yardage. also if i see something i just absolutely love but have no idea what i will do with it...i will get a fq...if i see a fabric and think of a dozen things to do with it i buy yardage...fq's just allow us to accumalate more variety with less expense...like buying charm packs so you can have a piece of every fabric in a collection-often 40-50 different fabrics...a charm pack or other pre-cut gives you all of them instead of buying one or two at a time in yardage.
all that being said...i very seldom buy precuts...but once in a while they turn out to be just what i need.