Why is it important to use parts that are machined from billet rather than cast from molten alloy?

Think of it this way...all metals have a grain, just as a tree or piece of wood does.  Melting a metal down and poring it into a cast ruins all molecular integrity of that material.  It is the equivalent of making sawdust from a tree and molding it into whatever shape you want it to be - like particle board tables, sheds, etc.  So, buying a cast part is lot like buying a particle board body.  Don't you want the body of your guitar to be a solid grain wood?  Isn't that best for tone?  Same goes for the metal parts on your guitar.  When a part is machined from solid billet of brass, aluminum, etc - not only does it keep its original integrity, it also carries out the vibrations better.  You know there are no other additives (they add glue to make particle board, they also use chemical additives to make cast parts!) that will reduce the quality even further.