This is basically a repeat of Freezy KO's post above, but...
People complaining about the fact that MK8 doesn't use the screen well aren't complaining because of a novelty horn, they're complaining that they paid $100 extra for hardware (or got $100 less in base hardware value or capacity as a result, either way you look at it) so that there could be a novelty horn. The same reason why Microsoft is being excoriated over and over and over again for charging $100 extra for their console for stuff that amounts to "that's a neat little novelty".
Of course there's Nintendoland and Game and Wario and off-tv play and yes, menu screens and maps can be handy, but if you want to have an argument about whether or not the Gamepad has in aggregate been a good value--I'm guessing you feel that the answer is yes here--then you're already admitting the problem was never the horn to begin with, but rather the perceived value of the Gamepad, so why pretend it was about the horn to begin with?
The horn is symbolic of the fact that not only do non-Nintendo companies not seem to get anything out of the Gamepad, most of Nintendo's internal teams don't either. The horn is symbolic of the fact that the Wii U has been rejected by the market at large by a number of reasons that ultimately come down to people not seeing the value of the product, and Nintendo's response is to say "The Gamepad is great. We're going to release software that really leverages the Gamepad" and then to proceed to release a release that has the Gamepad as a black screen, a release that uses it as basically a secondary info screen to offload HUD stuff onto, and their only long-term horizon that uses the Gamepad well is the prospect of NFC, which other companies have done without the Gamepad for several years now. That's the issue.
No one actually has a problem with there being a function to make a funny noise. Little goofy things are fine. No one blames the team for not using the Gamepad better, no one is saying "Mario Kart really should use the Gamepad for XYZ" because no one has really any great ideas for using the Gamepad, which is the actual issue at hand.