> Very few source code diffs scale the source code length itself exponentially or something equally naughty so its always safe to commit/pull source code changes and automerge them, not so much schema changes.

Awesome counterargument. However, in the same breath: what's the story with migrations? In RoR they're ordered by date, so what happens if two developers make concurrent and conflicting schema changes in their respective branches? That merge will silently fail, a false positive. At least a merge conflict throws up red flags, causing some human has to look at what is going on.