I'm surprised that they didn't find some scientific/artistic optimum for that, like they did with the Acme colors (which are picked by someone vastly superior to normal users who would just screw things up, if I remember correctly).

I read that too, it is supposedly invented by some great designer, which is, well, controversial, when you actually look at it ;)

After careful mathematical analysis, which this comment box is too small to contain, I've determined that the optimal number of spaces for a tab is precisely 2​e. Unfortunately, this turns out to pose certain practical problems. I imagine Plan9 came to the same conclusion and realized there wasn't a usable mathematical justification.

That still doesn't rule out evaluation by a proper artist. We just have to wait until s/he gets out of the Tufte Ashram(tm), bearing the answer on two golden ratio tablets, lettered in Helvetica.