A blogger once posted that she didn't wash her brushes. When they got too grubby to use she just tossed them and bought some more. That makes me cringe to my very soul. The price of the brush is irrelevant; if it's a good quality brush and you take care of it, that brush can last you many many years. Using dirty brushes can make you makeup look runny and awkward, can make you break out, and is just plain nasty. When you wash your brushes you're removing old dirty makeup, skin cells that have settled, and bacteria that can grow on your brush. Robyn posted a study of what grows on unwashed makeup brushes here, if you don't believe that bacteria shit.
I wash my brushes once a week. It's important that you do the same, even if you use it once during the week and it's barely dirty. After every use I run the brush over a baby wipe to get off any of the gunk that I can.
Please, for the love of god, wash your brushes. You can save money and your skin.
On a much lighter note, if you've ever heard the song December, 1963 (Oh, What A Night) by Frankie Valiie and the Four Seasons, my middle school band teacher is the guy playing drums and singing main vocals. I work with him at a music camp. We tease him endlessly about it, although it's pretty awesome. He's a fantastic drummer. You can watch him singing here.
