I think the point is, there'll be a certain amount of agility and flexibility built into the security systems, because while it's not possible to know exactly when it'll be needed, it is known that it'll be necessary sometimes. And people sign up to that.
So for example if you're in a palace protection role, you'll know well in advance that when a sitting monarch dies, certain role holders have to cancel all holidays and root canals, miss their kids concerts and the like for a couple of weeks. The same would apply if a less foreseeable event happens, like one of them gets held hostage or something. Bye bye to your fortnight in Spain, even if you were due at the airport tomorrow morning.
What that doesn't mean is that the capacity is available to spunk on things that aren't an emergency. A good security officer might think yep, I'm ok with a couple of all hands on deck, never mind your personal life type events every decade. Especially if they're obviously unavoidable. But if people are being expected to make those accommodations more routinely, particularly for something that clearly isn't an emergency, that's when there's a risk they burn out or get pissed off and leave. And why wouldn't there be more such requests, if this one was facilitated? It's just not a good idea.