We also don't allow them in our projects: everything that you want integrated into the main repository has to be rebased on the main repository branch on which you want it integrated.
This is to keep conflict resolution with the developer of the code, the one who actually know how to fix the conflict, rather than with the integrator.
It keeps integration cost very low and also keeps at linear history, which is again perfect for keeping features (consisting of multiple commits) together, which is again for for all kind of other things like bisecting and reverting.
Not so special this one thing :-)
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