You need this!
I don't know why I didn't buy this a long time ago, but I'm glad I have it now. Inevitably, without a cover, your bite valve is going to want to go where you don't want it to go whenever you set your pack down. The cover keeps you from having some dirt or bacteria with your water, it's easy to take off and on, and you don't notice it when it's hanging there during your hike or bike ride.