Dan Counsell of Realmac Software explains why it's so important for iOS app developers to use good screenshots:
“The simple fact is that a customer’s decision on whether they will download an app is mainly based on the icon, rating and screenshots. The name of the app and description [are] secondary, and most of the time not even taken into account. Potential customers look at these elements to try and work out if the app is worth their time and money, and this all happens in a matter of seconds.”
He goes on to give some excellent tips that may help them get a leg up on the competition. The bit about using all five screenshot slots is probably the easiest one to adhere to, and sadly there are developers out there too lazy to do even that much.