There is a degree to which scale makes it necessary to develop these sorts of projects. Losing 1% of productivity in a engineering group of 10 people might not be a big thing, but at 1000 people that's 10 full-time people worth of productivity you're losing. Dedicating 1-10 people for a few months or a year (or even two years) to remove that 1% productivity loss is clearly worth it.
Documentation costs productivity to write, but when there are many people who would be made more productive from it, it makes sense to do it.
I think "slack" is the right way to think about it. It isn't a free-for-all - the business still needs to run - but there's enough space to explore, to rewrite, to document, to polish, and so forth. That's where the magic happens - the unplanned and the unexpected things around that which you thought you were going to do.
