I thought the idea of these weekly challenges was to do something original for the comp, not to dig out stuff you had lying around
Basically yes - but if you don't have time to do new stuff and have something old already why not use that?
Ideally everyone would do their stuff from scratch, but one week is not that long time if you have a full-time job, wife and a small baby - for me it means that I don't have time to do ANYTHING weekdays/evenings/nights, and weekends are also often fully booked ("quality time" and all that crap like relatives :)).
IMO even if ppl use older stuff they have it might give other ppl fresh ideas how to do (compo related) things?