Personally I like the delay, but a preference to set the delay (0 or higher) makes sense to me.
This feature has gone back and forth a bit. Here are a couple old discussions on this:
http://forums.omnigroup.com/showthread.php?t=3963
http://forums.omnigroup.com/showthread.php?t=4180
Clean up also affects the Inbox, which not everyone wants to have cleared at the same time that their completed tasks disappear.