FAQ 20.9 - Addendum - why are 'a', 'an' and 'of' bad in name/insult strings?
Because people will be able to use those words as the name of the item or mob. If you have a sword with a name string 'sword of truth', you can do commands such as 'get sword', 'wield sword', or 'drop truth'. You can also do commands like 'get of', 'wield of', or 'drop of', because 'of' is part of the name string. The same problem also arises for mobs.