I've seen some maps with storyboarded warnings before double-speed sections (such as "Straight to Video" and "Help Me Eirinnnnn!!" and kudos to those that give us said fair warning, btw). One could suggest simply requiring this on all future maps with double-speed sliders, but that still wouldn't solve the issue of warning for those present in maps ranked before this will have become a requirement.
I can also personally see how warning of an approaching spinner (especially ninja-spinners!) could be useful; even when I know they're coming, sometimes I forget exactly where or how long I get with the spinner, and screw it and the next several circles up.
However, I can also see people arguing about authenticity and the discovery of how new maps work, so forcing this as a requirement would probably make some purists angry.
I figure the best solution for this would probably be a toggle in the options menu.