The published way that Foraker works is that it stacks with other SR, so it DOES make a very effective defense against (cartain) magical attacks.
This does not make the Forsaker immune to magic - there are many magical ways to still affect him, and he is still vulnerable to other attacks, of course.
I believe there was a tiny blurb in an errata, or FAQ, or was it a Sage Advice or just an email? that mentioned that Forsaker SR doesn't stack.
It's really quite simple - do you want the Forsakers in your game to be (effectively) immune to some kinds of magic, or do you want them to not get much benefit for all they give up (and giving up magic is probably the biggest PrC cost in the entire game of 3E, in my eyes)?