I've mentioned this previously, but will just add quickly:
Some projects I've been on (admittedly originating in the '90s, much, much later than the early UNIX tools), have had a only-allow-spaces-for-indentation policy. The rationale behind this was because content-based version control systems (such as Git) would treat any change in preceding whitespace as a substantial change to the file, and the spaces-only approach, the lowest common denominator, was the only choice that could make the file indentation unambiguous in the face of hashes such as MD5, SHA1, SHA256 etc. (Things like "diff -b" meant that some earlier source control systems could ignore changes in whitespace.) While there are other minor benefits to a non-TAB approach, my (not so humble) personal opinion is the elimination of hash ambiguity is the strongest argument for a spaces-only indentation policy in the long run. cheers, sur-behoffski [etc...]
