*Some* editors? It's *both* emacs and vim and it's done as a default in those because it makes navigation easier. Representing 32 consecutive columns as spaces is annoying. To turn around Kenneth's argument, in this day and age when every editor can handle tabs very well, is there any reason to use spaces slavishly for columns?
Every reasonable editor can handle tabs fine and set 8 columns as default, but every reasonable editor lets you change the default, and every programmer has their own preference.
Tab width is the one issue that I have never seen resolved when trying to come up with consensus coding standards in a development team. The only solution that works is to have source files use all spaces and no tab characters. Another possible solution is for the developers to use one editor or editor configuration for editing source files for the project, and a different editor or editor configuration for whatever else it is that they do in which they are used to their own tab setting.
If you push it I suppose I could figure out how to set tab width in emacs local to perl-mode. I don't know if vi users can do that. If they can't it would be very annoying for their editor to have the "wrong" tab width. Setting perl indentation to 2 vs 4 is much less of a problem to general use of the editor.
-- sidney
