Well, I think the new Macbook 12" was more of a showcase for new things Apple wanted to test the water with and see how popular they were. Making an "Ultrabook" for the purpose, rather than putting it in a "mainstream" Mac like the Macbook Air or an iMac made sense. (If it was a total flop, at least it wouldn't destroy a hugely popular product line.)
I actually bought one of these (in space gray and the higher-end configuration) to try using it as my primary work computer. I had been using a 17" Macbook Pro from mid 2010 before this, so I knew it wouldn't have worse performance than that. Typically, I leave it docked with an "Ultrastation" USB 3 dock made by J-Create, so I'm using a pair of full-size displays, regular keyboard, mouse and wired gigabit Ethernet connection with it.
When I want to take it someplace, it's just a matter of unhooking the USB connector to the dock and the power connector to the USB-C port on the side of it. Then it's as easy to take around as an iPad.
Having used it for a while now? I can safely say it works just fine for things like Microsoft Office 2016 (which I'm in all day long, especially for Outlook), flowcharting with Omnigraffle Pro, photo editing with Preview or GraphicsConverter and web surfing. It starts to show it has a weak CPU when you start using VMWare with a virtual Windows 7 or 8 session -- but even that is usable, if not exactly "snappy".
Bottom line? I think it serves a niche purpose. You definitely pay a big premium price tag just for the fact it's so thin and lightweight, and performance suffers. But the really thin, light Windows PC Ultrabooks were trade-offs in performance like this too (Atom processors, etc.). I don't care much for the new keyboard design on it though, and can't believe Apple is trying to push that as the new standard Mac keyboard.
Say goodbye to HDMI and USB ports.
Oh but don't worry, you can swap out your display and storage drives to the USB-C drive, which also charges the notebook.
If the MacBook 12" is any indication of the trend we are going to be drifting towards, even with the "Pro" line, then say goodbye to ground shattering performance. We can just use an Intel processor that's slower than a 2011 MacBook Air, as long as our device is super thin, incredibly thin and very innovative.
I feel sorry for people who bought the 12" MacBook and use it for anything other than being on this forum or responding to emails. You dumped $1300 into a device that's slower than your iPad. Congratulations lol.