When clinging, there are many cases where you want to face the same direction as the wall for your jump, (climbing two block high ledges for example), and many where you want to be able to jump the opposite direction with as much momentum as possible (cheesing creepers, jumping between trees, etc)
Both are equally useful, in many of the same scenarios, and I find it sad that I can only choose between one or the other for each gameplay session.

Mirror's edge (parkour game for those who don't know) had a keybind for quickturn, which I think is the best way to implement this.
for example, while clinging, press x to turn around.
[edit] it would also be cool to enable this on ladders

Wait. I didn't realize you could turn with the mouse while clinging.
This is now irrelevant. my apologies.