The metric system is supposed to be easy to use in part because it is based on the base-10 number system, and has prefixes for small powers of 1,000. Some measurements don’t seem to take prefixes that are too small or too large, however. The measurement of length, the meter, seems to use prefixes going increasingly smaller as needed, such as micrometers, nanometers, and even picometers and femtometers in quantum physics. (They aren’t common, but I have seen the words.) However, when have you ever heard of megameters or gigameters? Why do we say our planet is roughly 150,000,000 km from the Sun instead of 150 Gm? Likewise, volume is only measured in milliliters and liters. Granted, it’s rare that you need to use a measurement smaller than milliliters, but kiloliters and megaliters could come in handy, for instance, for describing large bodies of water. Why even have these prefixes if we’re going to neglect them and use a large number of digits instead? It’s a waste of a well-designed system.
I think megaseconds and gigaseconds would be neat, but nobody else seems interested in it.
I’m surprised you neglected to mention one of the most egregious inconsistencies in SI prefix usage: the base unit for mass is kilogram (not gram), and although you’ll easily find masses presented in grams, milligrams, and micrograms, it’s quite uncommon to see any prefix higher than kilo- attached to gram.
I remember learning as a kid that the “base unit for mass is kilogram, not gram,” but I never understood what this means.
Take the “kilo” off of “kilogram,” and you get gram. Add “kilo” to “gram” and you get “kilogram.”
Take the “kilo” off of “kilometer” and you get “meter.” Add “kilo” to “meter” and you get “kilometer.”
Why call the “meter” in the latter case the “base unit,” and “kilogram” the “base unit” in the former case? What work is this concept of a “base unit” doing?
-FrL-
Minor correction - in laboratory pipettes, microliters are fairly common. I’ve also seen deciliters used (not in the lab), but infrequently.
As to the rest of your question - damfino. I have also wondered about this.
$1,000 is a kilobuck.
It doesn’t mean anything for any practical purposes. The kilo is what is defined, and the gram is just 1/1000 of a kilo, rather than the gram being defined and the kilo being 1000 grams. Big whoop.
I learned two different metric system conventions - mKs and cgs; Meter, Kilogram, second and centimeter, gram, second.
At one time I could have even told you the reasoning behind having 2 systems (convenience, I think), but all I recall is having problems that stated “Express answers in cgs (or mKs) units.” Ergo, dynes rather than Newtons.
Making scientific calculations a damn sight easier. SI base unit - Wikipedia
Why couldn’t the gram have been defined in this way, rather than the kilogram?
If a kilogram is the mass of one liter of water, isn’t the gram the mass of one mililiter of water? (Maybe not, correct me if I’m wrong.) Then why not have simply defined the gram that way?
It doesn’t make much difference at all for most purposes, but it would have seemed to make more “sense” in a sense.
-FrL-
It’s even more egregious because the SI unit of mass (the kilogram) is defined in terms of a standard cylinder that’s kept in a special climate-controlled vault in Paris. There’s no reason in the world why they couldn’t have made the standard cylinder have 1 gram of mass. It’s totally arbitrary.
More Wikipedia:
Believe it or not, the metric system is used to measure beauty as well. The basic unit is the Helen (after Helen of Troy). One Helen (Hl) is enough beauty to launch a thousand ships.
A millihelen (mHl)is sufficient beauty to launch one ship.
If someone is really ugly, they have sufficient beauty to launch a rowboat. 
Megagrams are used, albeit under the name tons. Not only that, but gigagrams are called kilotons and teragrams are called megatons. I intentionally neglected to mention mass because of this complication. If someone would like to describe why we add the prefixes to ton instead of gram though, I’d be quite interested.
To answer the original question, any of the units can take any of the prefixes. There’s no reason why you couldn’t speak of a gigagram, a terametre, or a dekasecond. It’s just that people don’t happen to do this in common parlance, mostly because they have a certain intuitive grasp of a certain number of the prefix-unit combinations and find it easier to parse “a billion kilometres” than “a terameter.”
We do? (Isn’t ‘megaton’ in the context of explosions slightly different from the actual measurement of mass?)
I thought about that in repect to some of the really large and really small prefxes, but some are used for one unit and not another. Everyone knows that kilo- means a thousand and a kilogram is a thousand grams, so why don’t we talk about kiloliters?
It refers to an explosive force equal to what one megaton of TNT would cause. Come to think of it, I don’t think I’ve heard of kilotons or megatons outside of this context, but what on Earth would we actually measure with such terms? Why not call it a 5 teragram explosion?
In this metric system-using country, deciliters are fairly commonly used, for instance in recipes.
There’s no reason we couldn’t (except that a kilolitre is more commonly known as a cubic metre). I’ve definitely seen hectolitres and I think also megalitres.
And while we’re at it: 1 cubic kilometre = 1 teralitre.