The desktop is the one place where I don't need wireless, I don't care how big, old, or ugly looking my hardware is, and what I bought 10 years ago still works perfectly fine today.
The Magic Keyboard seems designed for using it from the couch. Any other use case seems better served with a "normal" keyboard.
Beyond that it's a bit cleaner on the desktop. I don't mind the look of a wire (actually I've never really used a wireless mouse because I don't care about the look, and the convenience isn't worth the money for me), but for example right now I have a notepad laying ontop of my wire, behind my MBP, behind my mouse, that's weighing down on the cable making it heavier to move the mouse. So I have to put the notepad below the cable, and then grab it from under the cable when I need it and this repeated throughout the day for anything I want to put on my desktop that has a mouse + charger cable crossing it. Ridiculous first world problems that I wouldn't spend money on solving, but if my mouse blew up today and a genie asked me if I wanted a wire on my free replacement mouse or not, it's clear what I'd choose. Another is when I disconnect the mouse on my MBP (hooked up to a keyboard, mouse and screen at home) because I want to plug in my camera or my phone to transfer some files and I only have 2 USB slots.
Anyway again I'm not one to have a wireless mouse but there's definitely people who benefit in a desktop environment and I'd wager to say it's not a small niche, either.
I widlarize mine though :)
It's looking a bit grubby now so I'll probably buy this new one to replace it.
I had much worse experiences with Apple's wireless keyboards when they first came out (pairing problems, latency, etc); but this seems to me like a technology that's pretty mature and "just works" now.
Also small arrows is a great idea: you can navigate through code without moving your hand to the right.