The files in question define tests for our hardware. In hardware testing, false-passes are the ultimate evil. A false fail can be dealt with (you just get someone to fix it), but a false-pass can make its way into production, un-noticed. Therefore, we'd far rather break modules that are currently seming to work, then to leave them with potentially hidden problems. Adding "use strict" is seen as a bit of low-hanging fruit.
--Dave Opinions my own; statements of fact may be in error.
Re^2: imposing "strict" on an included file
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Two reasons off the top of my head: it would require all the files to be touched (and touching a file is, itself a risk); but, more importantly, it would only work for existing files. One possibility to avoid that issue would be to grep the files at the start of the test and die if "use strict" isn't found. But a simple way to say "do_strict $filename" would be more elegant.