Document specific unit preferences

Question

I work on documents in Metric, US, and also column inches using Picas and inches depending on the client. It would be highly beneficial to have the document keep the units that the layout was originally created in. An example: If someone creates a document that is A4 size and builds all of the grids and so forth with metric units, it would be much easier to have that set as the default for the document, rather than having to guess which measurements someone might have used for their grid system or layout.
Likewise all of the styling would work the same as well.

Answer 1

Evening Peter,
So that's one vote for implementing the Metric system for type (myself) and one vote against?
Rather than using fractions of an imperial inch with the archaic quarters, halves, eighths, sixteenths and so forth, the metric system is all natively divisible by 10 (as a feature). This makes conversion super simple.
By "industry" I'm guessing you're talking about US dominated Desktop Publishing and not printing as in real printing with presses and letter forms. Sheet sizes are available in a variety of measurements overseas, as are press forms. More information about that is widely available in Google (and outside the range of this unintended discussion).
Unfortunately for your argument much of the world is in favor of the metric system. Since typography predates America, it's a bit presumptuous to assume that the rest of the world should use an American Inch based system as a "standard"; an American imperialistic viewpoint on the subject definitely. Much of the world would likely disagree.
While you're almost correct in your assumption that "type has always been spec[ifie]d in points," unfortunately the definition of a point has changed over time, and points weren't always the postscript points you're likely familiar with. That being said, it was Adobe who decided to use the 1/72 of an inch rounded postscript point for type in the first place, deviating from the standard American point a 1⁄72.27 of an inch. for ease in math for their users. Typography before computers was math heavy, and after all computers are supposed to make things easier. Most art majors aren't mathematicians. Before DTP point size wasn't something people had the luxury of changing depending on the printing system they used. They bought their typesets and used them.
While I realize that Adobe adopted the postscript point system back in its early beginnings, there is no reason (now) not to support the metric system for type measurement (in addition to points). The world is a smaller place thanks to the Internet. Companies now span continents. There is a required conversion from points to inches to metrics required for people not familiar with the US based point system.
If the software were to keep the "mm" measurement instead of converting to points it would be an added feature and improve the learning curve for metric based countries.
Additionally it would make metric system grid based layouts much easier to utilize.
Having taught typography at a college level for several years with experience explaining the various type sizes with points, EMs and so forth, I can safely say introducing this notion to people who aren't familiar with the US system is quite tedious. Introducing points, picas, and EMs for American art students is tedious in and of itself. Would it make the application harder to use... no.
When using a grid system based on metrics, the hold-out inch based measuring system for point size is the only thing keeping Adobe from cross border perfection in terms of the user friendliness. That and my other feature requests about making the UX/UI more intuitive.
Best Regards,
-Chris