I support this idea, because I do believe that there are many songs which switch from very fast-paced, hardcore sections to calmer ones. Well known examples would be System of a Down - B.Y.O.B., La Roux - Bulletproof, Florence + The Machine - Drumming Song, Rise Against - Prayer of the Refugee, System of a Down - Vicinity of Obscenity, and many many more. Just go through your osu! library and you'll quickly find many songs which could probably have used this feature. But, then again, I can understand that larger changes, like from 9 to 5 would already be confusing as fuck while playing. That's why I suggest limiting by how much you can change it. I'd say by 2 points at max. So, if you have AR 7, you can use 9, 8, 6 or 5, but not more in any direction. I'd like that, slight changes like those can already influence how fitting the Approach Rate for a section is. I for example would have liked -1 on the slow sections in Drumming Song and Bulletproof. -2 would only be necessary in really extreme cases like Vicinity of Obscenity. We obviously don't need to go from 9 to 2.
EDIT: And at best, there should be a break between the approach rate changes, but this is understandably not always possible, so the AR-2-Limit must do. And I'd say that if there is a break, you should have a larger limit, like 3, which could be very useful when mapping medleys or other marathons.