I've never bought into the "you shouldn't judge a book by its cover" thing. The cover gives you a first impression. It's the cover's job to illustrate you what the experience is all about with a still picture/logo. When you're browsing for a book, the book with the nicest-looking cover is going to be the one that stands out to you the most and so it'll be the first one you reach for to examine. If the illustration fails to express to you the mood of the book you'll be reading then it's a bad cover. Same goes for games.
So yeah, cover art is an important part of game production. When I'm browsing games there's lots and lots of games to look at, and the cover art that tells me "this is the sort of game you enjoy" will be the one I pick up and examine. If a game has a really nice boxart, then obviously the developer cares about its aesthetics. If the boxart is cobbled together and half-assed, then that's probably the developer's attitude towards the game. Even a simplistic cover with a plain background and logo can be a good cover, as long as it still gives off the 'feel' of the game. Obviously there are good games with bad covers and bad games with good covers, but that doesn't invalidate the point.
TL;DR: It's the cover's job to give me an impression of kind of game I'm going to be playing if I spend my money on it so it does matter if it wants to catch my attention.