Well no it's not necessary, but neither is cardio to lose weight. However, think about it, for every pound of muscle you gain, that's a permanent extra 50 calories you burn a day. Plus gaining lean body mass will make you look a helluva lot better when the weight comes off, instead of looking malnourished or 'skinny fat'. Plus weight training builds grit, character and discipline, which really is the greatest thing about it. If you have the genetics to gain weight easily, man, take advantage of it, think of how much muscle you could gain, really...I wish I was like that sometimes.