Customize and :version?

If this has been discussed before (not unlikely), please forgive my bandwidth waste... I noticed that Emacs have a :version tag on `defcustom' and `defgroup' when syncing some mail-lib files. I think it's used to indicate in what version a certain group/variable was introduced. It raise a sort-of fundamental problems, should XEmacs custom.el support this? (I don't know in what way Emacs actually use the field.) How? It mixes badly with the packaging system and it's separate versioning mechanism. Why I ask: If XEmacs custom.el don't support :version (which is the current case), do you think it's good to include :version fields in FSF synced files in XEmacs just for sake of minimizing differences? I'd like to do that, but I guess it could cause problems if XEmacs at some point changed their mind about supporting :version.