There are other more "negative" reasons too:

The language doesn't have varargs (to say nothing of type-safe generic variable-type args), so you'd have to pass in a slice of references e.g.

    print!(..., a, b, c)

would become

    print(..., &[&a, &b, &c])

Which is not exactly sexy, plus all these references would have to implement a matching trait, whereas currently formatting different values can involve different traits depending on the format specifier e.g. a plain `{}` is Display but `{:?}` is Debug and `{:x}` is LowerHex. You can have a type which implements LowerHex but neither Debug nor Display if that makes sense.