Without aerobics, you won’t burn sufficient calories.
Without strength training, you won’t tone up and might lose muscle mass. This results in the “skinny fat” look where even though you’re small, everything jiggles.
Without flexibility, you could be prone to injury and you will not get those long, lean muscles you’re looking for. Flexibility also improves your range of motion.
EVERY area of fitness is important! Incorporate them all into your weekly workout.