How often are you at a hotel with ethernet but no wifi?
Also even if that did happen what's he gaining with the wifi router that he couldn't use his mobile signal for?
Well, honestly... the OP asked a question, and I provided an answer. Whichever his reason, I don't have the slightest idea.
However, on a more personal level, I actually had to stay oversea at some hotels in Asia where they had just Ethernet, no WIFI, and those were in places so far out that I didn't even have any phone signal there. Luckily, the stay didn't go over a week.
You'd be amazed what you can find traveling oversea...
Why does my PC have to be pocketable? Are your speakers connected through airport pocketable? I forgot to mention that I can also access any standard USB device directly on the phone using usb-on-the-go feature. So that's one more option, using industry standard connection protocols instead of proprietary ones.
Well, a pocketable router beats having to lug along a PC everywhere, even though I do carry my laptop everywhere with me. Problem is... I know there are people who travel around with only their phone and tablet, but no PC, and they'd find themselves stuck somewhere without WIFI, or any way to connect a USB device.
In such cases, the Airport Express is tremendously useful... if only for a few minutes.
This is not an Apple promotion or anything of the sort. It's simply because the Airport Express is the only one of its kind that I mentioned it. If you somehow know of another pocketable device that can take Ethernet and share it out to many other devices as WIFI, I'm all ears.