When you get right down to it, it's still just arbitrary stuff that amounts yo "about yay long", "about yay heavy", and so forth... but with a means of describing what we mean to people (or whatever they choose to call themselves) who aren't present and have no access to an official artifact. The metre is about a cloth yard (a very old "standard") with an excuse (one ten-millionth of the distance from the north pole to the equator through the Paris meridian... or thereabouts; we should really work on a better way of defining that at some point), the litre is close enough to a compromise quart, which can be sort of defended by monkeying with the metre (hey, a decimetre's about a hand, right? Clever of us to have thought of that), and the kilogram's a bit too big and the gram's a bit too small, but you can't get 'em all right all of the time. Defining them all in a way that doesn't rely on artifacts and/or special moments in time is a good idea, but it doesn't make them any less arbitrary. There is nothing fundamentally more scientific about SI.