What do you want mundane's role to be? How do you want them to contribute to the game? Do you really want it to be, "I'm useless, but I survive to take a share of the treasure and XP from the people who actually contributed to our success"?

If so, then this is a good plan.

That having been said, I actually like the idea of giving muggles get huge, huge bonuses to saves (see Paladin), AC (see swashbuckling?) abilities like Evasion, etc. I just think that should be the icing on the cake, and you need to focus first and foremost on making them actually contribute to the game.

Of course, the same could be said for low-floor casters.