I'm surprised some people don't understand the need for a 42 sprocket. For me it's so that I can run a bigger front ring,say a 34,but still have a bail out gear for steeper bits. For most of my riding I think I'd find a 30 too small. Ideally I'd have xx1 and with the 10 sprocket have a 32 or 30 front,but I can't justify the expense. At the moment a new works components front ring and a oneup rear sprocket has cost less than £100.