In reply to Rowan Morgan:
To be blunt, I suspect yes, you are just being silly (joke, your words... ) I'm sure that quickdraws are tested as complete units, including the rubber keepers, as well as, as separate components. If there is any effect at all from the compression it must be insignificant if the whole unit falls within the huge safety margins climbing kit is designed with. If you think about it, the sling is going to get totally compressed against the carabiner anyway, and to a much greater degree when your weight is hanging on it, with or without the rubber keeper. Worry not! a non-issue.