At first glance it appears that Epicurus made a significant improvement by adding the third category of "unnatural." Having just necessary and unnecessary, for me, makes choices and avoidances rather black and white. The third category allows more room for personal nuance.
With two categories I think the tendency would be toward choosing the necessary and avoiding the "unnecessary" desires. This feels to me like a rigid sort of virtue ethic. With the three categories, the middle one (natural and unnecessary) becomes what I like to think of as the "sweet spot" where we make our most interesting choices. This is what puts the "pleasure" in "pleasure ethics."