The MOE is not "lighter". In fact, most quality handguards, 4-rail, 1-rail, or smooth, weigh less than most plastic handguards once you take the full mounting system into account. Stock barrel nuts and delta rings are heavy, relatively speaking.
1-side or smooth tubes do start out life lighter, but as you add rail sections they add up very, very quickly. This is due in part to the fact that in the mounting surface area you now have twice as much metal as you would in a dedicated 4-rail setup.
This is where, provided you don't want or need QD, the Mount-N-Slot products are great as they eliminate the need for redundant metal such as the rail section and tend to be lighter overall than a throw-lever mount because it has less moving parts.