Flat UI means less code and easier maintainability. It holds up better across a wide range of sizes. It makes it easier to have a style guide in code and create reusable modules. This allows the design team to focus on the bigger-picture user experience instead of perfecting button gradients.

High information density is great in many applications, especially interfaces supporting complex tasks for expert users. However, it takes a lot of work to design a great high-density interface. I feel like most of the high density interfaces I see are more of a case of the design or product teams not being able to make hard decisions about what's really important.

While some flat UIs certainly go too far in the other direction, I firmly believe that minimalism encourages good design by forcing a conversation about what information to prioritize. Especially for the mainstream consumer web, this is almost always better for everyone.

Finally, I don't appreciate this kind of attack on my profession. Good design is far closer to engineering than to taste. I don't think I'm hoodwinking people into hiring me.