I don't see a ton of 12.9" iPad Pros out in the wild, but I know a lot of people that have them. I think it's what ericwn said above--it's just not as much of a grab and go device. This is the main reason I have been sticking with the smaller Pro models.
I think the crazy thing about the different iPad sizes is how much they change the equation in your brain as to what device you need to bring with you. Here's kind of how it went for me:
Old school 9.7" iPads (maybe like iPad 2 through Air for me): Took them a lot of places with me if I were planning on just entertaining myself for a couple of hours while carting my kids around to stuff or if I thought I would be doing some emailing or browsing while I was out, but if I really needed to get work stuff done, I'd bring a laptop instead. iOS just didn't do enough back then for me not to. Eventually I started defaulting to iPhone plus laptop instead of iPhone plus iPad. At one point, I didn't even own an iPad at all for about a year.
iPad mini: Had one of these for about a year. Made a really good portable e reader and internet connected tablet, but couldn't justify owning it anymore once I started buying the larger model iPhones. I know the screen sizes are FAR different than what people make them out to be, but I would still opt for the iPhone more often because its screen was "big enough" to cause me to just live the iPad mini at home. Heck, I've read several full books on my iPhones, even going as far back as the tiny iPhone 4 screen!
iPad Pro 9.7": This was starting to get into the sweet spot for me more because of advances in iOS than the actual screen size. I never liked the concept of carrying around at 12.9" iPad Pro, so this model was a dream come true for me. With advances in iOS 10 and processing power, I started being able to convert most of my workflows to iPad with this one, and started carrying an iPad with me out of the house again. My main complaint about it was the cramped typing and screen size.
iPad Pro 10.5": This checked the typing complaints off my list and had noticeable screen upgrades too. Any time I go anywhere where there will be time for me to sit down and just "do stuff" in iOS, I always bring my 10.5" Pro. I could never go back to a 9.7" iPad or 7.9" mini at this point, and I this is the iPad model I've most taken out of the house with me on a regular basis. It's just an all around great machine.
The only iPad in that list that I ever flat out stopped being able to justify owning was the mini. It just doesn't have a good place in the lineup anymore in my opinion. Furthermore, the iPhone is probably still my main, most used, most versatile personal computer, so the iPad is almost always going to play second fiddle to that.
