Roasted turkey, covered or not?

After my mother made a great, moist, nicely browned turkey in her roaster with the lid on throughout the cooking process, I dragged my speckle-ware lidded roaster out of the basement. Tonight we had a hotel-style turkey breast, with garlic and marjoram, because I couldn't find my sage, kept the lid on the roaster the whole time. I made a broth with the neck and giblets, added mushrooms, finally milk and flour, and it made fabulous gravy.

More importantly, the breast was lovely and moist! Is there any down side to always using the lid on a roaster, assuming the beast fits?