About an "only class" - bonus ... for more role playing and better pvp

A lot of players like to play a special role, like a mage or a fighter.
First like to use magic only, second don't like to use a lot of them.
(For my part, I prefer to be a paladin, involved in both of them )

What about a bonus of 20% - 40% on defending(!) spells
for players just using glyphs of a special character?

May be a mage who don't use any fight skills or a fighter who don't use any mage skills.

The bonus could go down for every skill used from another skill tree ...
So a fighter could use a few mage skills (healing maybe)
a mage could use a few fight skills (shield maybe)
and not loosing the whole bonus.

As I started to use magic in this game I learned that there is nearly no chance to be a mage only
A lot players wrote about this, too...
But a mage or fighter "only" could be a great part of this role playing game.
So why don't give (magic) only players a bonus
Mages only +20% - 40% more shield power
Fighters only +20% - 40% more spell resistance

This would give us three great things:
1. a real roleplaying factor
2. thinking about specializing your skills
3. an approach for the pvp
The bonus would make it better to defend against high PVP's(PK's)
Why? If your shieldspell is 40% better in game you have time enough to run if you don't like to fight
And it's uninteresting how much skills a pvp has

And if a PVP uses magic only?
It grants him also better defending - not higher attacking points

Thanks for reading my thoughts and may be consider them in the further polish plans