I think the general idea is not to have one just for the sake of having one, but when used in conjunction with relevant relapse and disability to determine progression , (and yes, for me new disability showed my doc there was a need to switch treatment before I had my last MRI) it can be an important tool.

But it is just that...a tool. Not the end all ,be all of MS diagnosis or treatment. They are only a snapshot in time, what is going on at that very moment. Some lesions disappear and they can take quite a long time to appear in the first place.The lesions may or may not correlate to your symptoms.

All they actually provide you with is an idea of what's going on at the time. So they may help you decide if a drug is not working at the moment , determine that there is nothing beside MS causing a problem or help the doc in diagnosis, but can not predict anything, as I think we all understand. No doctor or test can predict the future here.

Ultimately, you decide when or if you have one.