"Cover art may never be as powerful as the music itself ...": I should hope not! It's all about the music in my world. Cover art should be the icing on the cake.
Covers that convinced me to listen before I knew the artist or was otherwise familiar with the music: 0.
Covers that influenced (i.e. never convinced) me to buy an album: maybe a handful and none that I can remember.
Covers that have etched themselves into my memory: several Cat Stevens albums; Dark Side of the Moon; Desperado; Jazz (by Ry Cooder); a few by The Moody Blues; Winelight by Grover Washinton, Jr. and many, many more.