Like Rose, I make banners for my novels, but usually not my one-shots, and I still get my regular crew of readers reading both. It shouldn't matter much either way. I will say, though, that when I'm just scrolling quickly through the Most Recent page, stories with banners catch my eye more than stories without. I could see stories without banners getting lost in the shuffle unless they're updated frequently enough to be on the Most Recent page a lot. I have clicked on stories because the banners caught my eye, but I've also avoided stories because their banners made them look like something I wouldn't enjoy. That said, I always at least read the summary before deciding whether or not to click on the story.
To me, banners are just like book covers - when I'm browsing in a book store, a brightly-colored or interesting-looking book jacket will grab my attention and get me to pull that book off the shelves, but my very next move is to turn it over and read the summary on the back. If the summary sounds like something I might like, I open the book and do a little skimming. If it doesn't, I put it back on the shelves. I'm the same way when browsing for fanfics to read.