I agree with you that most often the number of files increases more slowly than the size of the filesystem, but there are cases where this isn't true.
News is the one example that everybody knows.
True, but for a good news server you generally have to tune a lot of things anyway. And any decent news server maintainer should be able to tune a filesystem appropriately. The problem is that a default has to be best for most people. Linus pointed out to me, probably correctly, that one of the reasons for our lousy performance on the Mindcraft benchmark was probably related to bad defaults for the filesystem. And there are going to be far more file servers and web servers than news servers used by users, so it makes sense to make the defaults be optimized for the more commoon usage scenarios.