Often doctors do wait for two distinct flares....but it may well be that the MS specialist will decide, based on your test results (both lesions on the MRI and also o-bands, combined with clinical signs) that there is more than enough evidence to label it "definite". And it may well be that you've had a second..or more..episode, but haven't realized that, in your past. The doctor should take a thorough medical history, if your current neuro hasn't done so.
Besides, the newest recommendations for the drugs all suggest that the earlier you start them the better, to try to ward off increased progression. Just one lesion in the wrong place can lead to serious disability; why risk that, waiting for that second flare to appear?
You don't have to take the meds for life .. folks do stop them. But if you stop them, you risk the progression increasing. It's not like some meds -- that if you stop them, you'd do damage to yourself, or risk killing yourself, like would happen with insulin, or heart meds...but you do risk increased progression.
There are lots of oral meds being developed. Hopefully one of them will be found to be as good as the current meds, or better. Or even better yet, they'll find a cure for this disease, and we can all stop the meds altogether!