I don't even play CtB but I definitely support CtB-Diffs being rankable, so I'd like to adress the question why CtB maps should not be available for standard play as well.
As was mentioned before, CtB would use horizontal spacing and probably use rather large spacing/jumpy patterns to make it more entertaining. A few people have argued that mostly or even exclusively using horizontal spacing would be lazy since exactly the same patterns could be achieved with osu! mapping.
There is one problem I see, though. If you keep the horizontal spacing (x-coordinates) and add vertical spacing (y-coordinates) as well, you get even larger spacing (Pythagorean theorem - or just common sense). So, with the already high horizontal distance snap, this would result in ultra-high spacing maps for osu! mode. Sure they would be "playable", but really not all too fun for the majority of players.
Also, it would be pretty hard to create patterns that play out nicely in osu! from a base of x-axis spacing - basically the only patterns where hitcircles would not be positioned at the same angle would be those patterns that don't use 100% horizontal-only spacing in the CtB-diff (Streams for example). This is because every time you use 100% horizontal spacing, it leaves a fixed percentage of that distance to y-spacing if you obey distance snap, resulting in patterns repeating the same angle over and over again. Wow, that sounds confusing... Did anyone get what I mean? Probably not lol I'll see if I can create a picture or two for illustration later on xD
I think the reasons why standard osu! maps don't usually translate into good/challenging CtB maps and can even contain impossible patterns have been sufficiently discussed.
So a osu! -> CtB transition usually results in boring/too easy maps and a CtB -> osu! transition would usually result in boring/annoying patterns and awful spacing. I think that's reason enough to justify the need for CtB-only diffs.
As was mentioned before, CtB would use horizontal spacing and probably use rather large spacing/jumpy patterns to make it more entertaining. A few people have argued that mostly or even exclusively using horizontal spacing would be lazy since exactly the same patterns could be achieved with osu! mapping.
There is one problem I see, though. If you keep the horizontal spacing (x-coordinates) and add vertical spacing (y-coordinates) as well, you get even larger spacing (Pythagorean theorem - or just common sense). So, with the already high horizontal distance snap, this would result in ultra-high spacing maps for osu! mode. Sure they would be "playable", but really not all too fun for the majority of players.
Also, it would be pretty hard to create patterns that play out nicely in osu! from a base of x-axis spacing - basically the only patterns where hitcircles would not be positioned at the same angle would be those patterns that don't use 100% horizontal-only spacing in the CtB-diff (Streams for example). This is because every time you use 100% horizontal spacing, it leaves a fixed percentage of that distance to y-spacing if you obey distance snap, resulting in patterns repeating the same angle over and over again. Wow, that sounds confusing... Did anyone get what I mean? Probably not lol I'll see if I can create a picture or two for illustration later on xD
I think the reasons why standard osu! maps don't usually translate into good/challenging CtB maps and can even contain impossible patterns have been sufficiently discussed.
So a osu! -> CtB transition usually results in boring/too easy maps and a CtB -> osu! transition would usually result in boring/annoying patterns and awful spacing. I think that's reason enough to justify the need for CtB-only diffs.