Let's put it this way.
A bad cover won't always turn me away from a book, but I am more likely to pick up a book with a nice looking cover including evocative art and design.
A great example is this:
The book is called Last Dragon. This cover is fantastic, and made me grab it.
That is not a very provacative title, and had it looked like this:
The chances of me seeking it out would be slim.
However, if it was the same book, and I already had it my hands, the typical fantasy cover wouldn't turn me away.