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Yes. Think you'll need one.
Guide on the left in the above photo is for rear mech. Upward facing one is for the front mech, right hand grove is for the rear brake hose. For mechanical brakes, you need the one with the same cable guide on the right. (Like the one from the synapse I linked to) So that the inner has a guide into the chainstay.
I'd tend to agree with the don't bother approach.
I'm tempted to sell my CAAd12 as I have a Isen on the way and going di2 with that. If you're interested I could part out the gearset (DA 9000 mechs, 685 shifters and appropriate calipers (post mount and flat mount) PM me if interested.
I don't quite understand, its been a long day at work. Looks like I'm gonna need one of those hey?