2. you count a design by counting the blocks required to build a circuit in the air
admittedly this is a better argument, though i feel it is arbitrary...
Well I think air is the less arbitrary choice, mainly because it is the most ineffectual and neutral block of the game and even lacks any kind of hit box, not to mention that most of your standard minecraft world is made of air blocks (little less than 75%). There are also other secondary reasons that go for convenience: It is completely transparent; it doesn't require breaking before placing anything else; you can move through it freely; and i bet there are more reasons I didn't come up with.
But you're right, in the end any choice is arbitrary and it all comes down to finding a convention that applies to the most cases and this is usually done looking for the most neutral condition.
Btw did I mention I love you for starting this thread? xD
No. Better learn them correctly from beginning insted of letting them be in the shades just like you.
but you cant say it is correct when nobody really knows why this system is accepted by so many... The reasons given differ from person to person... You saying i am in the shade is not an argument... i am merely trying to find a solid answer to why... without asking why there is no reasoning