It really depends. Opiates work for some people, with some conditions. People should have the choice, just like with MJ. For me, one opiate worked, and another didn't (and made things worse. it was Oxycontin, incidentally, haven't taken it in many years).
Also, people with chronic pain take opiate breaks for the reasons of tolerance you describe.
And yes, I've heard MJ works for many people (I've never tried it for my conditions, since it's still not legal where I live. Also, I tried it a couple of times in college and hated how it made me feel, no way I could work on MJ). But I'm very happy it works for your spouse and that she's able to obtain it. Chronic pain is brutal. If MJ is ever legalized where I live I might try it just to see if it works.