I think there is no downside to treating any disorder. And MS-related fatigue and sleep problems are disorders, identified, diagnosed, and treatable. If you have a disorder and don't treat it, you're denying that you have a problem, suggesting that perhaps it's a "personal weakness" and somehow you'll get through it, or will "get over your laziness", or somehow "work through it".
The sleep medication, and the fatigue medication, will improve the quality of your life significantly, in many instances. Sleep problems are particularly worrisome, as they can lead to many other long-term health problems. You're not doing yourself any favors by not taking the meds as prescribed.
Having said that - -MS-related fatigue is a challenge to treat. Not all meds work for everyone. You might need to experiment a bit to find one that works for you.