I'm not using one currently but wondered if I should ?

I use a 720mm normal (ish) mountain bike carbon bar (Pro Answer) with shimano SLX brakes & shifters . The straps I use are Alpkit with a 13L airlok dry bag . It sits on top of my cables away from the head tube and seems to stay put apart from a bit of bounce on tough terrain ... The bike is a Cotic soul and the cables are tight (but not too tight to the frame) around the head tube and seem very sturdy ... Hope this info makes sense !

You dont need one.

The advantages are it will stop your drybag rubbing against your headtube while riding (this will take the pain off) and its less messing about camp as you can quickly unclip and not have to mess around with straps

As Chew say's. Use the money saved to have a weekend biviing .