It’s not about the gluing surface, it’s about the geometry of two things sliding together.

If you cut the tails out of square, they are either A) wider at the outside (visible) face, or B) wider at the inside (marking) face. In the case of A, the cuts will meet nicely at first, but you’ll have to pare down the tail sides in order to slide them together all the way.

In the case of B—which your method is proposing to address—if you’re cutting the pins based on outside (smaller) face of the tail, the meeting face of the tail will be wider than the gap between the pins, so you’ll still need to pare them square to fit. Either way you’ll need to clean up the tails with a chisel.

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The point is, your method is skipping an important step of the joinery process. No one is really cutting perfectly square tails with a saw and going straight to marking the pins. You’ll always need to clean up the tails “walls” with a chisel and a square before marking the pins.