Quote from Keilis
All the time. If you can't appeal a potential reader with a full-blown coloured cover, how does the mangaka expect to do so in the story? Unlike novels, manga is told by its visuals, so covers matter much more.
All that says is that mangas with better covers put more effort into marketing their product, not necessarily in writing it (or even drawing it...the cover can look much better or worse than the art inside)
"It is those who know little, and not those who know much, who so positively assert that this or that problem will never be solved by science."