From a design perspective font icons are a very good solution when compared to sprite sheets or SVG. They are scalable, they load a single time, and contain nearly every iconic type of symbol you could want in a UI. That's not an anti-pattern.
An SVG is scalable, cacheable and can contain any vector art you could wish for. And it doesn't use characters to represent images.
This is fundamentally different from icon fonts. twitter icon or mailbox icon are not meant to be used inside of text, so they have no business in fonts.
But fonts have hinting, so they look nice at low sizes and low resolutions. This is something that SVG is still missing.
Was their font just poorly designed?
Think for a moment about what Web Font users do: sprinkle garbage throughout their pages; run JavaScript to dynamically load stuff (and possible destroy the privacy and/or security of the client); dynamically update the page — all when a simple, semantic image could have done the trick.
It's kinda insane, like rendering a page by loading JavaScript to slowly append characters to the DOM.
Why, then, does one see all sorts of strange characters on web pages which use icon fonts?