"the purpose of null is 'unitialized'"  Not in any of the books that I have read. 

If you are worried about performance problems based on varchar(1) and char(1) then there are other design issues in the database that are causing them, not the use of varchar in this case. 

After designing multi-terrabyte systems in Oracle for the last 3-1/2 years, I will stick with the principles learned there.  I will be glad for the partioning capability in the next release.