Personally I think that "most" players in osu! don't delete all the elements. Some just like they way it looks and don't go straight into #1 on super insane's, others simply "respect" the mapper's work and keep it even if it may be so bad "created", some people maybe don't even know how to do it, or don't even think about it.
I played without deleting anything for a year and was "happy" untill I realised that deleting/replacing some stuff can help me improve on some maps. Although in the end if you play the map 100000 times you can for sure play even blind and nothing will disturb you since you just memorized everything, but using "this" just makes it less frustrating. I am sure that this issue with deleting anything about the map itself mostly concerns only people who try to get higher rank / top 40 at least.
So actually there are for sure not SO many people doing that, but I guess for people like me and I guess it's the minority, it is very "important" to have such a "freedom" to delete anything, or replace with our own, of course except for music/video.