You can have regular plate or even warplate that has a +6 bonus, there is just no good reason to do so, you don't save anything on the cost, you don't get any "special" perks, it is basically worthless in RAW...
However, in a "build your own" type of world it works nicely, picture:
"You have been handed down your grandfathers Platemail armour, you have spent (30gp) money to have it altered to fit you perfectly."
After a few levels:
"Your (Ritual Caster buddy) enchanter has imbued the plate with the essence of the items you have provided, it wears a little lighter, protects a little better" (Simple Plate +2)
After a few more levels:
"Your Dwarven (ritual Caster Buddy) enchanter has reforged your plate with the knowledge of the Dwarves, it is stronger and more durable than before." (Dwarven Plate +4)
After a few more levels:
"Your God's (ritual caster buddy) enchanter has imbued the plate with some of His Divine Essence, it is hard as diamons and wears like a glove." (Exalted Plate +6)
There are differences between a regular +6 armour and a named +6 armour, both in abilities as well as level...