I think the whole "well you can listen to the hitsounds and compare that to the music" argument holds some truth but it falls a bit short. First of all, not to be rude, but osu falls miles short when it comes to syncing compared to other rhythm games; I doubt most people take into account drifting bpm's and most maps tend to over-quantize notes to the nearest 16th "snap." Not to mention that like 95% of maps labeled [hard] or higher are, by definition, over mapped since they contain hitsounds that don't necesarily go to anything in the music.
Also, adding such a feature wouldn't change what's required to play the map itself. Physically speaking, it doesn't make the map any easier to play. Also, everyone would be entitled to use such a feature so it wouldn't be unfair.
There is some validity to the position against this feature though. So if this can't get implemented for "during gameplay" use, I think this would be useful for viewing replays so you could go back and see what you were doing wrong.