While I can see why you might think they would do this, I don't think it would be a good idea. The whole point of the switch is that you can take all of your console games to go. Every game on the Switch must be able to be played when mobile, so that means developers essentially have to limit the game's graphics anyway, so any extra processing power in the dock would go wasted. All it would do is raise the manufacturing costs (and retail price, perhaps.)
The only alternative I can see is if they developed a method for the OS to dynamically adjust graphics settings when you dock/undock the system. However, that would rely on individual developers utilizing this correctly, as the most demanding visual aspects of a game vary widely depending on the game itself. Also, because there would be a reduction in quality when going mobile, this reduces the consumers enthusiasm about being able to take it with them. Every time you take it out of the dock, you would think to yourself, "Do I want to play in lower-definition and potentially deal with choppy framerate, or just wait until I get home for the best experience?" It would be worth the trade-off for me personally, but I'm sure some people would be bothered by the fact they are getting "second-rate" performance when they take it out of the dock.
Even though you may think of it as adding performance to the docked console, I think many consumers are more likely to perceive it as a reduced mobile experience. That's not the impression you want to give off when your console's whole concept hinges on playing the same game at home and on-the-go.