hongkongpilot, have you ever considered that the bloke whose asking for the swap may be commuting and the request submitted may be his only hope?
It's about 'shifting' the trip. I know for a fact if I did such a thing, I'd send a Groupwise to the person, explaining why I am asking for this as well as expressing that I'd be willing to give them a G either in the same month, or whenever they required one later on.
It's all about helping your fellow colleague.