I agree that it's a different concept Helen. I do stick by belief that it would be a very useful addition to Airbnb though. So let me give you a use case. Currently, I am trying to book a skiing chalet in the Alps and we are a little bit late making our booking so there is quite limited choice. I created a wishlist of 6 properties that were close but when I showed the wishlist to my wife she didn't like any of them. So the wishlist is in the bin.
What I do know though is that we have booked our flights so our dates are set and we know where we are flying to, so our drivable area is set. All we now need is for a new listing to "pop up" that will be our ideal listing - because it matches our search criteria. If something doesn't popup, we have a back-up reservation plan to use a hotel we have used before. I don't agree that booking on Airbnb is time sensitive - if we don't like the places we see, we simply won't book but will wait. We're not prepared to settle for something that's not what we want and feel like we are being time-pressured into accepted something that's not ideal. Holidays are precious and we want it to be as perfect as it can be. If we have to settle for something that's not ideal then the holiday is off to a bad start before it begins.
So what I feel I need is an alert just like the one that Jordan734 above has made. In fact I am trying Jordans tool and am hoping that something pops up by using it. It's a query string that has my criteria embedded inside it. Jordan has done a very neat job of analysing the need that is expressed by many people on this thread. The general consensus of this thread is that it's a good idea.