@fortknox; variants don't modify the base code, that was what many significant changes over the last 3-4 months have been about, however adding a variant, having games which run that variant, then deleting the variant, is obviously going to cause problems as there are games in the database which require variant code which isn't there.
If I include a variant by default and people run it on various servers, then I decide to remove it, that would mean every developer either needs to reinclude the removed variant or remove the games which used the variant. Not the end of the world, but a big hassle that can be avoided by only including decent variants we intend to keep indefinitely (which is a common sense thing to do anyway)
Deciding to change one is different, because if only the code changes it would be fairly seamless, and there is a mechanism for wiping a variant's cache and map data and reinstalling, however if a change did something especially odd like remove an existing territory that would require lots of database data to be altered in a tricky way.
It's just common sense to only include variants that are complete&tested, and aren't going to get removed at a later date