I've been seeing this argument a lot and it makes the assumption that allowing alternative app stores has no impact on people who choose not to use them. I don't think this is true.
An app developer may decide to only list their app on an alternative store because they pay lower fees and they can handle payments themselves. As a user you now need to choose between not using the app or giving your credit card information to the app developer you may not trust.
If you get value out of the current App Store rules, like the person you replied to, then there is absolutely a cost to allowing alternative stores.