I think the basic deal is that you get more latitude in post, but only if you are editing frame by frame. If you are applying edits to the whole take, then IPB is just as good for editing. Furthermore, IPB has more detail in each frame because it isn't reliant on each frame providing all the information for the entire frame. Many people have reported block artifacts with ALL-I, because it requires significant compression in each frame.
Think about it, truly uncompressed 1080, ALL-I should be (2M(resolution)*3bytes(24bit color)*(24fps)=144 MB/s=1.1Gb/s
The 5dmkiii max data rate is 1/10th of that, so it requires massive compression in each frame. IPB circumvents this issue somewhat by putting more detail in a key frame.