Back when Ranking Criteria revision started, we discussed many issues regarding skinning. Our intention was to make rules and guidelines in such a way that the skin would contradict with the player's skin as least as possible. That resulted in very long discussions most of which ended with "We cannot accomplish that, .osu file doesn't allow this skin.ini setting". This is the reason I consider this change as essential and we'd like to come back to the RC discussion.
HitCirclePrefix, HitCircleOverlap, ScorePrefix, ScoreOverlap, ComboPrefix, ComboOverlap are especially important to be changable in the .osu file because they result in things such as this.If player set that in his skin, you basically can't do anything with it apart from forcing the default skin as preferred which replaces the cursor, which is most likely the major reason people disable skins.
Another problem is that we want the skin to work for people with any "SliderTrackOverride", so we need to force people to set it in .osu file, which however changes behaviour of combo colours. If we want the track to be the same as combo, it won't work for people who have a skin that is using SliderTrackOverride, it will have only one colour, but to avoid mixing with map's skin, we enforce using SliderTrackOverride. Instead we should have a setting that allows us to choose one track colour (SliderTrackOverride) OR to inherit that track colour from combo colours. We can do it only by setting preferred skin to default, but that again replaces things we don't want to change.
tl;dr Everything that directly affects how skin looks during the gameplay should be possible to set using the .osu file without setting preferred skin to default.
HitCirclePrefix, HitCircleOverlap, ScorePrefix, ScoreOverlap, ComboPrefix, ComboOverlap are especially important to be changable in the .osu file because they result in things such as this.If player set that in his skin, you basically can't do anything with it apart from forcing the default skin as preferred which replaces the cursor, which is most likely the major reason people disable skins.
Another problem is that we want the skin to work for people with any "SliderTrackOverride", so we need to force people to set it in .osu file, which however changes behaviour of combo colours. If we want the track to be the same as combo, it won't work for people who have a skin that is using SliderTrackOverride, it will have only one colour, but to avoid mixing with map's skin, we enforce using SliderTrackOverride. Instead we should have a setting that allows us to choose one track colour (SliderTrackOverride) OR to inherit that track colour from combo colours. We can do it only by setting preferred skin to default, but that again replaces things we don't want to change.
tl;dr Everything that directly affects how skin looks during the gameplay should be possible to set using the .osu file without setting preferred skin to default.