I think "Never" is too strong a rule. I'd vote to leave some personal leeway around whether people check commented code in to the repository. The ultimate goal should be coder productivity, not "a pristine repository."
To balance that laxness out, make sure everyone knows that commented out code has an expiration date. Anyone is allowed to delete the commented code if it's been around for a full week and never been active. (Replace "a week" with whatever feels right to you.) That way, you reserve the right to kill clutter when you see it, without interfering too directly with people's personal styles.
