Thanks for the feedback. I agree there are times when you want to fill an icon. It’s not that we have a fascination with keeping them trasnparent as it is that we’re just using the icons as they are—with transparent backgrounds. That’s helpful when you want to use them against any area with a fill/background color.
Having said that I agree that it’d be useful not to have to place another icon behind to add a fill, and I’ll add a feature request to our tracker to add a property to icons for filling the hollow spaces with a background color, because it wouldn’t work to always fill with white in every scenario.