It's self-explanatory, really—it just feels like a massive oversight that slider head circles and slider tail circles can be skinned separately from the hit circles but not the approach circle. For example, in my color-coded skin HueShift, the hit circles are colored differently from sliders, but the approach circles can't be distinguished in the same way. I could only see it as a mistake, a careless omission to the customization capabilities of this game.
Imagine what you could do with separating slider approach circles from hit approach circles. Not only could you entirely change the color of the sliders, distinguishing them from the hit circles, but you could even change the shape of hit circles and hit circles only—Imagine having hexagonal hit circles without the added burden of clumsily-shaped sliders. I have found three such skins, but their sliders, consequently, look terrible. If slider approach circles were separated from hit circle approach circles, hit circles could be hexagonal and sliders can be circular, without worrying about what shape the approach circle should be.
Another reason I propose this change is, you know, just for consistency. Right now, as far as I can tell, all the elements of the Hit Circle can be stopped from being reused in a Slider. Don't want your hitcircle.png on your slider? Add sliderstartcircle.png and sliderendcircle.png. Don't want the overlay? The skinning system has already got you covered. But the approach circle? Not a chance. It's just a massive oversight, and it surely deserves to be implemented, even if not every skinner is going to be aware of it.
Imagine what you could do with separating slider approach circles from hit approach circles. Not only could you entirely change the color of the sliders, distinguishing them from the hit circles, but you could even change the shape of hit circles and hit circles only—Imagine having hexagonal hit circles without the added burden of clumsily-shaped sliders. I have found three such skins, but their sliders, consequently, look terrible. If slider approach circles were separated from hit circle approach circles, hit circles could be hexagonal and sliders can be circular, without worrying about what shape the approach circle should be.
Another reason I propose this change is, you know, just for consistency. Right now, as far as I can tell, all the elements of the Hit Circle can be stopped from being reused in a Slider. Don't want your hitcircle.png on your slider? Add sliderstartcircle.png and sliderendcircle.png. Don't want the overlay? The skinning system has already got you covered. But the approach circle? Not a chance. It's just a massive oversight, and it surely deserves to be implemented, even if not every skinner is going to be aware of it.