I use vim. When indenting with tabs, I can use "x" (delete one character) to remove a level of indentation. When indenting with spaces, I can't. The dedent command is more verbose than just hitting "x". If I forget when working in a file with spaces, then I have a three space indent. That off-by-one-character is harder to see and correct than a giant missing tab.

Why have one of the most common semantic units in source code (indentation) represented by four characters when there is an otherwise unused character that has already great editor support. Tabs work well in every editor I've used (except notepad.exe, which can't change their display width). Spaces only work well in smart editors.