The problem with that measurement is that you get extremely different numbers depending on what you're making. e.g. the density of memory is massively different to the density of logic.
There was a concerted effort some time back to formalize a standard measurement of process density using something like "million transistors per square centimetre" by a standards organization. (IEEE?) It wasn't a perfect measurement but it was a lot better than width. It failed so completely that I can't even Google it any more. The awkward name probably didn't help.
edit: it's "MT/mm2". Some people actually use it, but more in the informal sense that has the problem you espoused rather than the formalized one, which I still can't find.