The argument is that spaces are always spaces, and the text will always look the same to anyone who reads it. If tabs are used, the argument is that different tools expand tabs differently, and therefore, documents with tabs in them may not always look as expected. I understand this argument. However, spaces are not as convenient as tabs when editing code. If you are editing code, it is nice to have the various indentation levels of the code represented as tabs.
Tabs make it convenient to move over to the next indent level, or back up to the previous one.
When the tab key is hit, these editors add the correct number of spaces to the document to take the cursor to the next tab stop. The problem is that the editor cannot really know if what it is backing up over was typed as a tab, or was typed as spaces. The best solution in my opinion is to have the editor do what the user asked it to do with a minimum of guessing. This means that if the user types a tab, he gets a tab. If the user hits backspace, he gets a backspace.