In my experience with our app, prompting for reviews is worth it. Our average sales have gone up about 3x and I attribute it mostly to having more reviews spurred by prompting. Especially with a low number of users you just don't get enough reviews otherwise. Prompting doesn't have to be too onerous either if the app waits until it's been in use for a while. Also if it lets a user dismiss the prompt permanently how annoying can it be? If someone likes an app and uses it, giving a review is a significant way they can help ensure it survives. If it takes a little prompt to motivate them it's worth it for both sides. Prompts have gotten me to review apps positively in the past that I otherwise might not have bothered.