I get the feeling that I might be the only person on this thread that doesn't have a maths/sciences/computers background here. I'm going to be frank here: We need to snap out of the mindset that all of the data we're collecting is going to be easily expressible using modern scientific units and methodologies. If we try and cram everything into a small number of common units, without giving the users some method of expressing non-standard/uncommon/non-scientific values, we're going to have a massive database that is going to at best be cumbersome and at worst be useless for a great deal of information. Traditional Chinese units of measurement [1] have changed their actual value over time. A "li" in one century is not as long as it is in another century, and while there is a li to SI conversion, it's artificial and when we try to use the modern li to measure something, we get a different value for that thing than the historically documented li value states it should be.

There is a balance. The more flexible the parameters, the easier it is to put data in, but the harder it is for computers to make useful connections with it. I'm not sure how to handle this, but I am sure that we can't just keep pretending that all of the data we're going to collect falls nicely into the metric system. Reality just doesn't work that way, and for Wikidata to be useful, we can't discount data that doesn't fit in the mold of modern units.

Sven

[1] http://en.wikipedia.org/wiki/Chinese_units_of_measurement