Bullshit. I'm a Googler, and I'm 20%ing today (as soon as I finish breakfast).
There's lots of pressure to focus and do well on your main project, sure. But it's not external pressure, it's internal pressure. Everyone wants their software to be great. It's hard to detach for a day, ignore the influx of bugs and emails, switch gears, and work on something completely different. Most engineers don't want to spend the mental overhead.
But speaking from experience, when you do choose that path you get nothing but support.