Those planets typically don’t heave a breathable atmosphere, though. You pretty much need a large biosphere if you want to be able to walk around without a spacesuit. An iceball world or a barren rock probably won’t contain a breathable amount of oxygen in an otherwise mostly inert atmosphere. If you want to breathe pure carbon dioxide or get fried by nearly unfiltered UV radiation, though, they’d be great.









That does make sense when you need absolute precision like when doing abstract math. Otherwise you can just use whichever unit and number of significant digits you need and be precise to that amount. That’s what you do with imperial/American customary units as well; a 5/32" screw isn’t going to be manufactured to the precision of a Planck length; manufacturers specify their sizes to three significant digits of an inch.
Let’s say you have a machining project and your tools are precise to 0.1 mm. So you plan things out at a precision of 0.1 mm. It doesn’t matter that a distance is 17/38 cm exactly. It doesn’t matter that it’s 4.473684210526315789… mm. You can’t set the tool to anything better than 4.5 mm anyway.
Also note that the metric system doesn’t prevent you from using fractions. You’re perfectly free to work with fractions where useful. That’s just not how people talk about lengths because those fractions have no meaning outside your specific use case.