One other good reason to have a good sized stash is economy. Not too long ago you could buy Quilt shop fabric for 5 or 6 dollars a yard, and in a very short time (cotton crop/ drought/etc) that price easily doubled.There is no sign that it will come down in price..just keep rising..so if you have a good sized stash of basic blenders and backings, you can save yourself a ton of money on every quilt you make by using blensers or backing from your stash.
Also..some of the theme fabrics are here and gone in a flash..if you like it you had better buy it when you see it, because it won't be there when you are ready to make a quilt. Harry Potter fabric comes to mind..that stuff is selling for a huge amount on e-bay and other sites now..if you were a fan and wanted to "someday" make a quilt..you're either out of luck or going to pay big now. Other fabrics just disappear...never to be seen again..so if you like birds or cats, or whatever, and you see it and love it, better invest!
If you can afford it, and it's your hobby...you aren't hurting anyone...everyone needs a hobby and something that makes them happy...that's my story and I'm sticking to it!