This is also false economy, as you sacrifice half your farming area for less than a 100% boost in yield. Given most automation options effect a given area rather than individual blocks you'll achieve a much higher yield farming the entire area.
Actually, I don't sacrifice anything.
I have (typically) a row of pumpkins on the outside, to make jack-o-lanterns for light.
Then, rows of wheat, potatoes, carrots
Finally, a row of melons on the other end.
100% of the ground is being used.
It's just not being used for only one crop.
As for why is it done? It represents crop rotations. Heck, in real-world farming, using only food crops results in water table problems -- having a mix of native plants and crop plants helps keep the water table under control, to prevent excessive salt in the ground from a water table that is too high up.
EDIT: And I don't think you understood it right.
If all you do is spread one crop, you are sacrificing 5/9ths of your land for a 100% increase in growth. So your net production is 8/9ths.
But, you get your first produce faster. You get the results of your work sooner.
And, if you spread the same number of crops over more space, then because you are using twice as many chunks, you get twice as many block ticks -- another 100% increase.
So, 100% increase in growth from crop rows.
100% increase in growth from spreading your crops over more land.
Still think that dense farming is superior? As mentioned, put other crops in-between.