How can I avoid NULLs in my database, while also representing missing data?

Question

In SQL and Relational Theory (C.J. Date, 2009) chapter 4 advocates avoiding duplicate rows, and also to avoid NULL attributes in the data we store. While I have

Answer 1

NULLs are required - theres no need to replace them

The enitre definition of NULL is that its unknown - simply replacing this with arbitrary type is doing the same thing, so why?

For the comments below:

Just tried this - neither is true:

declare @x char
set @x = null

if @x = @x
begin
select 'true'
end

if @x <> @x
begin
select 'false'
end

I can only take this to mean that because null is unknown then it can't be said that it equals or does not equal - hence both statements are false