I've seen many people many times pedal this argument about how airport X is full at peak times and that it should therefore be allowed to expand as airlines won't fly - or will to only non desirable destinations - at off peak times.
Might I remind those of you who follow that line of reasoning that LHR is full throughout the day - whilst demand drops off a little at off peak times it certainly isn't that noticeable. If a slot becomes available at LHR the airline will take it regardless of when it is, often paying huge sums of money for them (United once bought 4 slots for £40m each). This demonstrates just how much demand there is to fly from LHR.
If an airline can only make a route work out of Manchester say by flying it at peak times, then whilst there is clearly demand for it, it is not deemed essential - otherwise the airline would take an off peak slot.