Using calibre's checkpoints feature, you can get a pretty good idea of what parts of the EPUB got touched by a plugin.
You can also gloss over the ToC, opf, and other xml-heavy things that probably don't have the changes you are looking for.
It's not quite concurrent versioning, but it is pretty darn handy to see the points where the editor or a plugin changed things, and what they changed. It's not even about reverting -- checkpoints are also an easy way to just run a global replace and confirm the changes, rather than slowly step through each instance.
Or even just seeing the checkpoint titles tells you what tools or plugins have been used!
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Yes, there is a utility to be found in having such a system (and there are multiple ways to leverage it). Whether everyone needs that, or Sigil needs to focus on providing that, I cannot say.