Why would one implement this instead of using Android's builtin widgets? Android apps have always felt better to me when they use the UI recommended in the Android style guide.
Because the built-in resources are very Google-brand looking. Big apps have their own style. They'll follow UI norms, but don't need to look like Google made the app.
Agreed. We already have icon placement in TextEdit controls via the drawableLeft, etc. attributes. And if your app doesn't have a theme font of some sort you are better off using the Roboto system one so you don't look strange for no reason.
I guess the only real use is for the icon font, but people were doing that already and it's best practice to generate one with just the symbols you need.
I don't think Android Bootstrap has anything to do with HTML5. It's a set of themes for native Android apps duplicating the style of Twitter Bootstrap.
I really can't differentiate between Disabled buttons and Rounded buttons. There would also be very little differentiation between the a basic button with no icon (because there isn't a good icon for everything) and the text input. Android design guidelines and 'language' is very different, and people expect the apps the buttosn to have some depth and clarity because of it. I, personally, would hate to have any of the apps that I use move to this style.
Responsive frameworks like Bootstrap are already designed to work across desktops and mobile devices including Android, so there would be less of a motivation to make it specifically for Android. This is a native app boilerplate that creates the look and feel of Bootstrap.
Because the built-in resources are very Google-brand looking. Big apps have their own style. They'll follow UI norms, but don't need to look like Google made the app.