Tufte himself acknoweledges that the rule has limitations:
“The principle makes good sense and generates reasonable graphical advice-for perhaps two-thirds of all statistical graphics. For the others, the ratio is ill-defined or is just not appropriate. Most important, however, is that other principles bearing on graphical design follow from the idea of maximizing the share of data-ink.”
(Visual Display of Quantitative Information, Chapter 4).
Given this, and the examples that Tufte presents, I think that spirit of the rule is best captured by intrepreting “ink” as “graphical elements”, and treating the rule as an suggestion to remove unnecessary elements (or parts of elements) from graphics, just as Strunk and White advise writers to “omit needless words”.
I don’t think that the ratio is necessarily useful as a quantitive measure, though I think that other quality metrics can be.
Interestingly, Tufte does refer to Strunk and White in VDQI, but only as a source for the quotation “No one can write decently who is distrustful of the reader’s intelligence, or whose attitude is patronizing”.