I'm sure Apple looks at the numbers of second batteries that they sell versus the number of laptops they sell, and it probably becomes very clear that there's no point in supporting such a small market segment. I'll bet it's a similar story with docking stations.

(Also, for what it's worth, I bought the docking station for my old HP Omnibook 500, and it turned out to have a roughly 10% chance of electrically frying its motherboard every time I connected it. After four replacement motherboards, I got them to swap it out for the newer Omnibook 510 and I gave up on the docking station altogether. I'm sure if Apple tried to build a modern docking station, they'd do one that actually worked, but it wouldn't be cheap.)