Ok. In the sense of an RFC "SHOULD" is more appropriate.
Tabs give me the possibility to indent code as I prefer it (Tab=4 Spaces) without forcing anyone to view it exactly the same way. People use indentation from 1 to 8 columns, why should i have to force anybody to like 4?
And about the 80-character width: You'll need a 48 character statement on a line after 4 8-Space-Tabs to hit 80 characters. For two reasons this cannot/should not happen: You should always break lines after about 30 characters for readability AND indentation deeper than 4 levels should indicate that a subroutine to split up the nested logic is needed.