The problem with using the "extra_ship" name is that the modders and shipset builders need to be on the same page. What I mean is that you don't want the player to have to rename the ships to work in a mod. Also, what stops a shipset builder from adding what he considers a supermassiveheavyorbitalassault carrier as extra_pic1 and the next builder making micromines as extra_pic1?
That's why the standards are there.