A significant amount of the writings from the ancient world in particular are lost to us. Are all the writings that survive useful simply because of that fact, or is there any that we could describe as being useless? I was just curious.
The utility of primary sources is determined by whether and how historians use them. A historian should regard no primary source as useless except with regard to their immediate needs. It has often happened that primary sources overlooked by one generation or school of historians become essential to another, for example the Annales school looking at sources pertaining to material culture neglected in the predominant approaches of the period that focused instead on politics, diplomacy etc.
Burke, Peter. The French Historical Revolution: The Annales School 1929–89, (1990)