Thats probably true.
But i am pretty sure the only reason as to why the drug dealers and users use metric is that it is more easy to deal with from a math perspective.
And as long as you know what a gram is in the different substances you are good to go, as that's what most get dealt with over here in metric country at least, and even if you move up in larger quantities due to heavy use or you set up shop, then you just deal with the kilo suffix witch relate to grams pretty easy.
Kilo = wholesale / kingpin normally
1/100 of a kilo, one or more 100 grams = sale, you might want this person to move up to kilo territory not least if he come calling several times a month.
Grams ( 1/1000 kilo ) you would want to leave those people to the guy mentioned above or people below him, this mean heavy traffic to a address, and you really dont want that to be your address.
Or if you are the user this is what you would want to stick with or spiral out of control and out of help / reach of people that care for you.
In general you do not want out of control drug users in your life, no matter if you are a dealer or a relative, do not underestimate the destructive power of such a person and the substance he/she take, its just a big hornest nest to stick your fingers into.
For imperial i guess that when it get lighter than a ounce, you then go so and so fraction of a ounce, and then the math get hard unless its 1/10 of a ounce ( 2.8 grams as i recall )
you can of course go street and use the "eight ball" measure, whish if my ex - stoner memory serve me right is something like 3 grams.
EDIT: I think if you go to the GYM and lift weights you are familiar with kilos too, even if you are in the Imperial world.
You will know a kilo are a pretty manageable weight, a child can lift that before it can walk.
put a few 10 kilos on a dumbell and you start to need bigger upper arms and pecs if you are lying down, pass 100 kilos and you will start sweating pretty fast even if you have thin arms and this are your max or you are a high power athlete that do many reps as you are there for the distance events.
Add a few 100 kilos to the bar, and you will need to be a frightfully large person depending on which muscle group are the target.