Having lot of respect for Damian Conway I would like to point out that almost all useful editors allow you to define the tab length. Be it 2, 4 or 8 spaces.
And that's the one and only the reason why tabs are evil. Tabs are fine if they act the same in every editor.
Suppose you have a 80-column coding standard. You have a 3 character indent as standard. You set your tab width to 3 spaces because you indent with tabs, and you have a subroutine containing a loop, and an if statement in that loop. Inside the then block, you have a 65 character line. In your editor, the line ends in column 74, well within the boundaries.
Now the next person never uses tabs, and uses an editor (s)he didn't customize to your habits. Hence, the tabwidth will be at 8. Now your 65 character statement ends at column 89, violating the coding standard.
That's why tabs are evil. Because you do not know how where the tabstops of the next person will be. Spaces are predictable. Tabs aren't.