I am looking at the same option, 128GB with LTE. Mostly I want it because my current machine is aging and while it's still worth something I reason I can sell it for about the same as what I'd pay for the iPad Pro. I'm all for going more mobile and shaving some weight and having data wherever I am is a huge thing for me.
The others are optional, I will get them both because I am an artist and sketching my ideas is necessary before I start to make whatever it may be. The keyboard, why not, it's always good to have another functional option available as well as protection for the screen.
For the things that people are persistently whining about, I don't even understand 3D Touch and I don't plan on getting a 6S so I won't experience it for along time, I doubt it'll become a life tool and I'm sure I can live without. Running OS X? that's a dream. I don't think anyone is really thinking ahead, the OS is just software, it can be updated and perhaps Apple will create some sort of iOS iPad Pro centric feature that allows for a file browser and App developers are going to make the more desktop-esque apps we all love. They do it all the time with older devices that don't support some certain feature.
If what Apples claims are true about it being faster than most of the mobile computer that shipped this year (or whatever they said, something like that..) then there is a lot of wiggle room. Someone mentioned on another thread that their old Intel Duo Macbook can run Final Cut. I think it's a pretty fair assumption that if a 7 year old macbook can run a desktop app well enough to be useable, the iPad Pro can do similar or better performance with an iPad optimized app, it just doesn't exist yet.