But if you use spaces instead of tabs, indentation will also align. And you don't run the risk lines will be pushed over the 80 char limit if someone uses different tab settings.But using spaces you are forcing the others to use for indentation the same number of spaces you've chosen, while they may prefer to use different settings.
Think for example of a visually impaired person (or simply a tired night coder) who wants 8-spaces indentation, who deliberately sets her tabs that way, to obtain what she wants regardless of the indentation habits of her colleagues (and still preserving their freedom to nest as deep as they prefer).
By using spaces for indentation, as you suggest, your are forcing her to use your own indentation settings (which could be even a single space for that matter), which is not kind at all. Furthermore, any decent editor permit you to (automatically) convert tabs to spaces (the amount of them you prefer), while the opposite is not (always) true.
In a word, tabs are portable, while spaces aren't (so tabs are not evil ;-)