Well of course the MRI is a diagnostic tool, not a treatment tool, so I personally don't see the need to have repeated MRI's
EXCEPT in these situations: a) there is some doubt about your diagnosis, and you want to be sure that it's MS, and not some other disease mechanism going on
b) if you start a new treatment plan, it's a good idea to have an MRI when you start, and then one a year or so into the new drug, to see if there are any significant changes (for better or for worse)
and there may be some other distinct reasons...
I had an MRI 14 years ago when I started Betaseron, and then one about 2 years later, after I had a small exacerbation. That's the last one I had. When I moved here 9 years ago my family doctor wanted me to see a neuro for a base-line exam. I thought he'd want an MRI, but he was content with seeing my "old" ones. He said something like, "...you have MS, there really is no question that you have MS; MRI's are very expensive, and all it would tell us is what we already know...so I'm not going to order one.."
As for lesions: MS-related lesions are much like real estate. The old saying that real estate agents use, when asked about the value of property, is "location! location! location!".
In MS terms: you can have lots of lesions, and if they're in relatively insignificant areas of the brain, they may cause little or no physical damage that you can detect. Or you can have one or two lesions in a really significant place and you'll suffer a lot of physical disability. Early in the course of the disease, lesions can seem to "disappear" on an MRI. But often the damage is already done to the myelin sheath, so you may not experience a decrease in disability. Or this has happened too: someone has died and the autopsy has shown all kinds of MS-related lesions -- but in life they had none of the symptoms of MS. location, location, location.....
And remember, too, that you can have an MRI, be moved out of the machine, be put back in immediately and have another MRI -- and they might well appear different, as the focus is a bit different, you've shifted a bit, your head has moved, whatever.
Obviously -- since I haven't had an MRI in years -- I have no idea whether I have a lot or a few, or whether they've faded or reappeared. For that matter, I've never known how many lesions I've had..my MRI's have only stated things like "...too numerous to delineate..."