We were talking about this during a meet-up of an OMS Circle. Disease progression does not always manifest as new lesions on an MRI and conversely, new lesions on an MRI do not always manifest themselves as new symptoms.
So quite why neurologists use the presence of new lesions on an MRI as indicative of disease progression and thus drug choice, I know not.
IMHO our biggest enemy is stress and the emergence of new symptoms, even temporary, has the power to cause us damage. Your friend here is meditation and mindfulness, staying in the moment with the breath. I speak from experience here, with recent emotional trauma causing a significant increase in my symptom level; when I practice mindfulness and meditation in these times (which is a difficult challenge) I find the symptom level decreases.
The same applies to new lesions show up on an MRI in those people who have in their experience recovered. The Profession wrote a blog post on this subject, which sadly not available on the "new" website but his reply to the OMSer who posed the question was quoted here:viewtopic.php?f=10&t=4930#p31311