Most of what you say is sensible and reasonable. But what Ken says is "If you're considering a D300, forget it. Get the D90 instead. ".
To say as Ken does that the IQ is the same is misleading and simplistic. In many situations the IQ is not the same. If the shutter speed is in the danger region, about 1/15" say, then you probably do NEED mirror lock up, and the D90 will give you a soft image, whereas the D300 will give you a sharp image. And as you indicate, if you need fast focus and high FPS, then the D90 will not cut the mustard.
It is of course up to you to decide whether or not you will use the extra features and hence whether or not they are worth having.
Unfortunately Ken's 'essay' was (unlike your sensible post) rather simplistic.
Actually his essay makes sense if you realise that the target audience is happy snappers who want to take pictures of their family and friends. Nowt wrong with that, but that is not the same as bird and nature photographers.