Well, theoratically Catherine could have been "appointed" principle hostess at this event, too. This has nothing to do with "Prince of the Blood". If that would play any role, Edward and Alexandra would have lead this invitation to BP (Pcss Alexandra was in fact so "invisible", I haven´t seen any footage of her at this event so far....The reason for that is of course by the media who decide who´ll be covered and who is not. And obviously elderly cousins of HM in the back rows of the monarchy are not of particular interest these days...).
In fact, they didn´t, but obviously, as someone claimed, it was Prince Edward and the Countess of Wessex.
Before I read this thread, I thought it was an "equal rights" host/ess occasion, just doing this engagement together. Well, now we have what we so often see on royal forums in the net: the usual discussion about who is "more important" than the other one... I think this says much more about us, the "royal watchers" than the Royals themselves...!
You can say what you want, but as Pr. William´s wife, after Camilla, Cath. Cambridge will be the next Queen after her and by that will always play a prominent part in and for the royal family. And yes, the Jubilee will be about the Queen and her family - which also the Dchss of Cambridge, Sophie Wessex or Sir Timothy Lawrence are part of - and I´m very certain The Queen also regards them as such!
Whatever the spin may have made in this, the host was Prince Edward on behalf of the Queen. Catherine was subordinate to Edward, just as she was subordinate to Anne at a previous engagement.