each placeholder should ideally be grouped with the feature it's most closely related to - `%clearfix` is mainly for layout (it's generic layout, but has a very specific effect), so people checking out `core/_layout.scss` would likely want to see it there. %nice-type is for typography, which in many frameworks has its own partial (with all the heading styles and body text styles, typographic vertical rhythm, font size modifiers, etc), so that would be a nice direction to go. %button_personality is very specific to buttons, which we have a designated partial for.

Having a `placeholders.scss` or a `mixins.scss` file feels like having an `objects.js` or a `functions.js` file in javascript - people generally want to know "what does the stuff in this file do?" rather than "what language feature does the stuff in this file use?"