I disagree. I currently work at a startup with 10% time, and barring rare "the servers are on fire!" events, we're pretty religious about doing it. Even when we skip it, it's credited so we can take it later (arguably better since you get longer stretches of it).
Like most startups, we also operate at breakneck pace, we're not exactly sitting around twiddling our thumbs. This strikes me as the same as "I don't have time to..." excuses people make in their personal lives. If you value it enough, you will find a place for it.
It's all about how much you actually value the 10%-time, or if it's just a recruiting/PR gimmick. If the company approaches it from a "it would be nice if..." angle, it will always find a million things that are higher-priority than 10%-time. If you approach it from the "this is a critical part of company strategy" angle, you will find ways to keep it sacrosanct.