Your intelligent merge tool has access to the file history. If one user modifies a layer, and the other one squashes them down, in the merge you probably want to apply the changes in that order, even if it's out-of-order chronologically. If the file format has a full edit history baked in, great; even more info for the intelligent merge. Maybe the in-file history can even be kept in sync with the repository level history.

In the current ecosystem you probably need locking for your sanity, but some day software will suck less.