That's less a fault of our interface devices, and more that the airlines are not in the business of giving us the cheaper fares. ;) I'm sure they'd be happiest to suggest fares that were cheaper than competitors, yet 3x the price of a different day's fare for the same trip.
Not on Adioso, which is why I use them. You can choose a city (with multiple airports) for the departure location and something like "about X weeks/months" for the travel duration. And then drill down, since they display a range of dates.