Here we have the opposite: Twitter decided that tweet bodies should perform a marketing function. They sacrificed utility for brand cohesiveness. I understand the motivation, but there's a middle ground that serves both purposes: use your own custom font for the UI and use a system font for user-generated content. Gmail does this: the UI is Roboto, while the default font is "Arial, Helvetica".
I'm sure I'll get used to it, but eh.