They are of little value outside use as ammunition for the Rock-It Launcher. Milk cannot be consumed in the game, as only empty bottles are found.
Milk bottles can be found in numerous places in the Capital and Mojave Wasteland (particularly in houses) and certain merchants will sell them as well.
A substantial stack of bottles can be found in Allistair Tenpenny's personal suite, an apparent allusion to the eccentric behavior of millionaire Howard Hughes, who subsisted in self-imposed seclusion solely on chocolate bars and milk in the later stages of his life.
In the Mobile Base Crawler from Broken Steel, a set of ten mini jugs can be found between two desks along with a garden gnome posed with the 10 jugs as bowling pins and a 4-ball as the bowling ball.
In Fallout: New Vegas there are many traders that live in buildings that own Milk bottles.
In many instances of both Fallout 3 and Fallout: New Vegas the milk jugs appear in crates much like the milk that was delivered Post WWII by the proverbial Milk-man, further promoting that particular era as "the" background theme for the above Fallout games.
Also found in most building across the Mojave Wasteland and Capital Wasteland.