that is completely untrue... pedaling smoothness is massively important for optimizing power efficiency. every single cycling coach will tell you so. most of them have specific advice on how to pedal smoothly. there is a reason that single-leg drills and whatnot were created.
FWIW you don't need rollers to practice smoothness. I find it works best on a trainer when you can really break it down and focus on only pedaling. you can also do the aforementioned one leg drills.