In reply to jezb1:
Ha ha, yeh I know it's a bit fat!
Main downside to a fatter rope is weight. As a scrambling rope will typically be shorter in length than a climbing rope, the weight saving isn't so noticeable.
A couple of nice advantages to the fat rope are:
1) Will usually be harder wearing.
2) It generates a greater amount of friction, thus easier to control. This is a particularly useful trait for a scrambling rope as it's more likely to see action on direct belays around spikes of rock.
I have 4 short ropes that are about 10.5mm that I occasionally use for scrambling and I can literally hammer them with abuse and they take it really well. I've also 3 x 9.1mm climbing ropes that see occasional scrambling action and these need very careful handling on direct belays. The level of control is acceptable, but the rate of wear & tear is not and these are really expensive ropes.
Jez, you must have loads of experience with different rope types and diameters. What's your go-to scrambling rope?