Can we please move away from the "omg AR10 to AR5 on no bpm change just for the lulz is the worst!!!1!1" and get back to the actual point of why this was requested? This also is a big thing for Standard, not for Mania.
The changable AR would allow us to
support specific/special parts of a song that dont make sense to have a high AR on (for example, Image Material). Does AR10 make sense on dense parts of a 260BPM map? Yes. Does it make sense on a 28 BPM part? Not at all.
Lets say that there's a song that is halfbpm for a long time, but then ramps up to doublebpm. Having a lower AR would fit the first, slow part and having a higher AR would fit the later, last part. What to do? Low AR kills the fast part, High AR kills the slow part. Finding something inbetween doesnt work, that would mess up either part.
If someone would make a shitty map with shitty ar changes, then it would never be qualified. If it's unranked, then just revert the AR to a stable one.
Standard Maps are mapped for Standard, jesus. Stop comparing your shitty SV changes in a converted map to your awesome whatever else. We use SV differently.