During normal (non-disrupted) operations, almost all of the stands are in use almost all of the time. Yesterday, BA operated 630 movements at LHR and used 79 stands over the course of the day. That's called maximising the utilisation of your assets.
It would be hugely expensive to maintain a batch of spare stands to cater for the relatively few days per year (typically around 15) when things go pear-shaped.
Same applies to spare aircraft, spare runways, spare anything in fact ...