For years google (and many members of the design community) made a very successful argument that android apps should look and act like android apps, and iOS apps should look and act like iOS apps and web apps should act like web apps. Trying to achieve design consistency across platforms was going to annoy your users. Instead you should strive for _branding_ consistency across platforms and use native interaction patterns.
A significant problem for Android has been iOS designs just copied over without adapting to the platform. The apps look and feel weird. As a user I find them confusing and frustrating to use. However progress was being made and people were starting to understand that if you want to build for Android you are going to need to design for Android.
Material throws that out the window. It says it right there in the goals[1]: "Develop a single underlying system that allows for a unified experience across platforms and device sizes."
IF we take it as given that our apps should look the same on all platforms, then why choose Material? Because I know my customers are going to say: "We have this great iOS design sitting right here, that we have already paid to have built. Lets use that! Besides we don't want to re-code our iOS app to suit Android". Or, they will come up with their own cross platform design to "differentiate themselves" and stand out.
[1] - http://www.google.com/design/spec/material-design/introducti...