Indentation. Monospaced fonts allow much easier scanning of indentation levels. For Python this is crucial; for brace-delimited languages it's merely important for many of us.
I don't follow you -- `easier scanning of indentation levels'?
With proportional font, the left-hand whitespace used for indentation of your code looks exactly as with monospaced font. Only the text itself changes somewhat.
The width of tab/positions of tabstops has nothing to do with font itself; it's property (often settable) of the editor (or in rare case of the underlying terminal). May be expressed as `times the space width', but that's it.
Perhaps you mean aligning of function argument and/or parameters one below another, or indentation of parts of expressions one below another? That indeed can't work with proportional font IF the peers in your project use different font (size or shape) or tab-stop setting. Can't work with monospaced font either -- again, IF the peers on your project use different tab-stop settings. Which is prevalent, AFAIK. Some use 12'' netbooks, other use 23'' desktops. I've even known a guy who used a T221 [1]. One size does not fit all.
