I think both systems of units typical cases are dominated by human factors. Anecdotally, humans tend to use one of three scales for their preferred choice of measurement and they tend to be smaller-than-human, human, larger-than-human.

For example, the km. I see mm, m and km used quite often.. but the other bases are almost never used. A prime case for this is "what is the distance to the sun?" If you're going to round your answer then the most concise form is 150 Gm, yet most people will still say something like "150 million km" which is unnecessarily long and non-canonical, but is much more familiar to the human mind and makes comparisons to everyday experiences much simpler.

You can see this factor occur with just about every other metric base unit, even though the conveniently prefixed part of the system would be "better" for all technical definitions. Just as easily you could use imperial units and just overlay the metric prefix and scaling rules on top of them and it would be no more or less accurate or better than the metric system of units.

> Use them all you like. I won't.

I use whatever is convenient or common for the application.. if it gets complicated or I need a conversion, I use a unit-system aware language or environment to do my calculations. It really obviates the importance of the choice of unit and allows you to think about the fundamental problem more clearly along with many other benefits including conversion of units at any stage of calculation.