The answer would vary with each person, depending on his/her own situation. Realistically, how long would the yarn be sitting around your home waiting to be used? Having a stash sitting around is not such a bargain if the money you spend on yarn could be going toward paying down debt, saving you chunks of money on interest. Or, if you have no debt, the money could be put in a high-interest savings account, earning more loot for you and ready for you to use for any emergency that might come up.
If you're already financially set for life, then ignore the above considerations and buy it all.