I prefer to separate the debate about which system we should be using in real life from what we should be using in games.
What matters in games is different from what matters in real life. The real world's game scale is wildly inaccurate and inconvenient. People in a fight don't stand 5' apart. The things we carry aren't designed so that their weights make for convenient round numbers in pounds or kilos.
The numbers used in games aren't real numbers, but they are subjective correlatives to real numbers. We presumably have an idea how much 3 lbs. is, and the number is more convenient to work with than the 3.22 lbs something might way in real life. In games, more convenient numbers rule, because they are not meant to map onto real life except in the broadest kind of way.
I believe the solution to the problem of weights and measures is that only those relevant to the game need be dealt with at all, and of those numbers should be selected to be convenient to express in either system.
A 5' square on a D&D map is not a 5' square in real life. In game terms, it is equivalent to the 2 meters of Star Wars or the 2 yards of Savage Worlds. You could go with 1 yard/meter as in GURPS, but I think the people at PEGINC have a good point that about six feet is where player characters tend to top out, and to have the vertical measurement set so that a person of about six feet is still taking up only one square when knocked prone is probably a good idea.
I read above here, and elsewhere, that the German edition converts 5' to 1.5 meters, which I think is a bad idea even though it's more accurate on a real-world scale, because the real-world is irrelevant here. 5' = 2 m makes much more sense in the game because they're both convenient to work with and you can even convert between systems in your head. But better still is simply speaking in squares, as D&D 4.0 does.
1 square = 5 feet = 2 meters = 2 yards
Forget whatever the real world equivalents may be, and in most cases speak in terms of squares when you're not trying to give a sense of scale.
For weight:
1 kg = 2 lb
Again, not real-world correct, but the subjective correlative is close enough to equivalent and generally numbers convenient in one system are also going to be convenient in the other.
Distance:
1 mile = 2 km
Easy to substitute for those rare cases where it matters, such as Teleport spells.
Much harder to deal with are spells that specify things like "1 foot-cube of mud per level" because it's harder to convert them to mutually convenient numbers. But what are you going to do with the information about how many foot-cubes of mud you can create? Near as I can tell, what you really need to know is how much of game-scale real-estate the spell allows you to affect, and so spells like that should be changed to eliminate quantities with a granularity less than that of the game scale except where the Hardness and Hitpoints by Level scale is concerned -- and those kinds of tables need reformed.
Mind you, 4th edition seems to have gone away from a lot of these problems by eliminating sub-map scale granularity. But I'm still beating my head against the same issues in Pathfinder RPG.
