Right, so the question is "why not have side-by-side diffs". The fact that syntax highlighting would work is one obvious advantage. It also means that you can read the before/after code linearly without having to mentally apply the patch. It seems like an obviously better UI for understanding the effect of changes (although of course I don't have data to back that up).
I would like to see GitHub offer the option of seeing side-by-side. It can be helpful if you have a mess of tiny -/+ hunks right next to each other. But combined is definitely the correct default.
