I think it does matter. There was a 20 person limit for this event. The organisers were running themselves ragged trying to allocate places. It would have been so much easier if only the definites added their names.
Those that do not remove their names for non-attendance will be forever known as unreliable.
Unfortunately it happens all the time, people like the look of something and put their name down then something else comes up and they forget about the fact they are down for it. Maybe that's not the case here but it's what usually happens - or they do it deliberately for the karma nonsense in which case you'll never convince them to remove themselves. It would have caused headaches before the event for Trish organising it, but now it's done and dusted it doesn't really matter I'm sure.
The Midlands xmas party is the one I know it really matters for because if folks faces aren't on that one and it goes to 100 then they won't get in, so anyone putting their face on that one with no intention of turning up is actually stopping other folk from attending.