I brought Image Material into my post because it has a very clear change of pace and is widely known in the community. It's really fast basically all the time, but then there's this, comparably, extremely slow part. I took the 28 bpm from the ingame information, I didnt actually check what bpm it was at that very time.
It could be any other song, really, as long as it'd have a fast and a slow part.
If the song was fast all the time, you'd give it a fast AR.
If the song was slow all the time, you'd give it a low(er) AR.
If the song is a mix of both, then why arent we allowed to mix the ARs in a well designed manner too?
Edit: I guess songs like DeltaMax and similar would fit the example better. Going from AR9 to AR10 over the course of 2 minutes wouldnt hurt a map on that song at all.