Interesting ... that would be good, but you'd still need to be a little flexibile. For example, if I wanted to book a trip to PHX next weekend and I selected Friday - Sunday, I'd have to be willing to sacrifice my entire Friday instead of being able to fly out Friday night. Of course, given the ridiculously low airfare for such a trip I can get using the Negotiator right now for such a trip for next weekend, it's not a terrible sacrifice.
See
this site for a description of the Weekender feature that was offered (and apparently has been replaced by the Playtime Guarantee) that allowed you to select flight time windows, so for example on my Friday to Sunday trip I could have selected a 4-9 PM flight time window on Friday and the same for Sunday.