If you have ever added on to a home, or lived in a home that was added on to, you likely know exactly where the addition is. These additions always feel like additions. Adding on becomes a problem because you are building on top of something else, instead of integrating a new part. You are adding layers, not integrating features.
Integration versus layering: integration makes a good product, layering makes an average product.
Layering never gives you the ‘it just works’ feeling.