Spell users will get resisting spells for free, but the rest can also get a two times in defense if they plan their character well. Using RR as level 1 for everyone is a possitbility, but that it is equal easy to land a spell on the sect leader and his goons might not be desired.
Actually, you might want to do something here - still use levels for resistance, but do not assign resistance levels according to "experience", but according to scenaristic importance. So your goons will be level 1, but your cult leader will be level 10 - but only for resistance purposes. Skills, and skill development, is not correlated with level anymore.
You can even use different "levels" with that - a leader might have a high resistance vs mental magic, but a normal resistance to physical aspects (physical magic, poisons, and the like). A spy and assassin might have a moderate resistance to mental magic and a high resistance to toxins.
But when you get to this (resistance uncorrelated with experience, but correlated with role), you can remove level and assign a resistance modifier instead. Which allows you to keep the casting bonus - characters will be able to overcome resistance more easily with spell proficiency, but this resistance will still vary from individual to individual according to something that is more ingrained in "the way the world works" (scenaristic importance) than in practical and measurable values.
You still have to know what to do with spells that allow to detect a target's level, though. Does it measure field experience (DPs), or does it measure that "relative importance in the world" thing that can be thought as Destiny ?
And if you assign resistance capability to Destiny, you are just one jump away from allowing characters and NPCs to trade it for actual survival - escape certain death ? Spend destiny. The villain escapes certain death, but they use up something vital and, from then on, are closer to their demise (and so their Destiny has unraveled a bit and their resistance to aggression is lessened).
Probably does not work for all settings, but has a certain appeal for those where fate, or soul, is a significant parameter.