Good question Styx. I'm on the Imuran and my train of thought is much the same as yours. Maybe we'll both get an answer to your question.
The answer to your question is yes, Imuran does slow disease progression and is known to alter long-term disease progression -- in some people, but not all. It doesn't *stop* disease progression, and it doesn't help everyone. But studies have shown that, statistically speaking, it is beneficial in slowing the onset of complications requiring surgery, compared to any other meds available.
The analogy to its use in RA is right on the money.
I would also add that Remicade/Humira are also capable of slowing disease progression and not just masking symptoms (like prednisone) - at least in the short term (<5 years). Long term studies of course are still ongoing, so not as much is known about their ability to alter the long-term progression of the disease.