... unless it's primarily a partnership/corporate focused event, at which the public are an unecessary nicety.
I work at a charity and we do loads of mid-week events, as usually they're not aimed at the public or at individual donors. They're often about corporate fundraising, at maintaining/building partnerships in related industries/charities, networking, launching/finishing large projects, etc. Joe Public wouldn't really add anything to these events and would hardly ever even be invited.
I'd not be surprised to hear the Abberton thingy is the same. Though of course I've not read anything about it in any way whatsoever, so this is entirely speculation