Before built-in support for SVG button icons, developers used raster background graphics. The SVG graphics have their perks, but they don't become part of the button style: a style for a button does not include the choice of icon for that button. This means that changing the icons for a series of buttons using the same style throughout a file requires manually editing each button, rather than only the style. This inconvenience has been enough to keep many developers from adopting the new button icons in favor of the old raster background graphics, despite their otherwise inferior flexibility. If icon selection could be included in button styles, this would no longer be an issue.
One likely challenge to this would be how it works for button bars. All the buttons in a button bar use the same style, but we don't necessarily want them to all display the same icon. It would be helpful in these cases to have a style inspector with the ability to include or exclude various factors, such as icon selection, from a style. Alternately or in combination, it would be helpful to be able to apply more than one style to an object (and the order of precedence for those styles).