I realize this thread is WAY old, but I would like to offer another opinion that may shed some light on why northings,eastings are used in favor of x,y.
First, x,y is a rectangular system, Cartesian coordinates, and are ORDERED PAIR (x,y or x then y. X (being "a cross", actually goes across the page as the east west axis), Y as the north south axis. Y increases in the NE and NW quadrants, decreases in the SE & SW. X increases in the NE and SE quadrants, decreases in the NW & SW.
Northings and Eastings are just x and y reversed, meaning that they aren't an ordered pair...they are actually (y,x).
so why would we do this? Well, I would imagine it has a lot to do with surveyors and having to convert between rectangular coordinates and polar coordinates (r,θ) or (distance, angle). Remember that it is a rectangular coordinate system, therefore it is a RIGHT TRIANGLE, we can use Sin,Cos,Tan to find length of sides between coordinates, with the line between the two points being the hypotenuse, and one side being change in Y, the other change in X. so what side is adjacent and which opposite...well since in surveying lines are based off of bearings measured from north or south axis as zero always to east or west axis being the 90's (bearings never are greater than 90 degrees), the change in Y or the northing is always the adjacent side of the reference angle (the bearing angle). For instance a bearing of North 40 degrees East is measured from the North being zero, toward the east 40 degrees. Same for a South 40 degrees East bearing, measured from the south axis as zero toward the east 40 degrees.
But that doesn't explain why Northing, then easting or Y first then X.
Well, if we continue on, converting from polar coordinates (distance, angle) to rectangular coordinates always gives us relative coordinates, not ABSOLUTE. In other words it gives us deltas or change in X, change in Y rather than absolute coordinate values. This is important, but not nearly as important as the understanding of bearing definition as compared to the Unit Circle in mathematics. Polar coordinates with (distance, angle) are based off of the Unit Circle in trigonometry. In the unit circle in trigonometry, 0 degrees is DUE EAST and increases in a counter clockwise manner. Example, due north would be 90, due west would be 180, due south 270 degrees. You know this if you are familiar with autocad.
BUT...bearing angles are based off of North or South being zero and increasing clockwise or counterclockwise to the east or west. Many older calculators had functions of converting from polar to rectangular coordinates, but are based off of math and science using the unit circle from trig. Therefore, when using Sin of the angle multiplied by the distance of the line (sin θ multiplied by the hypotenuse length) results in the change in X rather than the change in Y. You must understand that the angle that the unit circle refers to is the complimentary angle to the bearing angle referenced (at least for northeast)
With the single button function a surveyor in the field could convert polar to rectangular or vice versa rather than doing separate calculations using sin, then cosine. Since calculators give the rectangular coordinate conversion as Y, then X, I would imagine many mistakes were made with applying the change in Y to the X coordinate and so on. It was probably easier for the surveyors to start using (Northings, Eastings) rather than ordered pairs to decrease the number of mistakes made by not remembering to put the Y value first then the X value in the calculator.
That's my opinion, based upon absolutely nothing more than seeing my own students making mistakes with their calculators and getting confused with X,Y and N,E.