The meaning behind "The Soul Is the Rock" is,your inner soul (everything spiritually) that makes us who we are inside and out,is our greatest strength in getting through life.
"Rain on the rock",represents the challenges and the other things we face in life and (as the old saying goes) "what doesn't kill us (inside or for real),makes us stronger".
When the "rain" ceases,the sun comes back out and/or life goes on.
I agree,it could have been put a simpler way but true songwriters make songs that make us think harder than normal...and that's a good thing.
Anyone could write a dance song or a (teen like) love song but,lyrics like these mean a whole lot more.
As for his not performing it,I think he'd simply say,
"I just don't at this time but if I get the inclanation (if he feels inclined to) I'll sing it".)