But if a startup wants to become a successful larger business (rather than just get acqui-hired by Google), then the code quality does matter. Once they've gotten paying customers, they can't afford to drop everything and build a reliable and scalable system from scratch - it would be cheaper and less risky to build on top of their original code.

Also, depending on the business, the MVP may end up being a large and complex code base that can't just be hacked together haphazardly. Not all startups build iPhone apps; some build on-line retail businesses or investment banks.