Firstly, thanks for the thumbs up, glad it amused you.
I wanted a good fit on the taper. If I had done as you said and cut the taper with the op in the first instance, I would have had to have left the large boss on to hold the blank to machine the reverse side. This would then have necessitated a further op to reduce the boss to the finished size. If you are suggesting that the whole of the front face could have been machined in one setup, then there would have been a danger of crushing the taper when machining the obverse side, plus this extra step would negate any advantage you were trying to gain.
Cutting the taper using the topslide was also made easier because the reverse side has the recess. It's quite a deep hole for a small boring tool, so any advantage in chip clearance because of the through hole, on this tough alloy steel was welcome. It gave me the advantage of not having to drill deeper than was necessary to give clearance for the taper thread, thereby potentially ruining the end of the remaining stock.
I see no logic in holding it in a four jaw to machine the rear. Clocking the back side of it would have been awkward to say the least. Then there are the holding marks it would leave on the diameter. The tooth thickness is only .188" thick, which would have needed some holding force in the four jaw.
You may think differently, but like all things in this game, you do what suits you.