It really depends on one's needs. I use my Pro as an adjunct to my Mac. I like being able to carry it with a Bluetooth keyboard when I don't want to lug around a 15 inch rMBP. I can write in Word, do some Excel work and then surf the web. With the video dongle I can even carry it in my briefcase instead of the Mac and still run a presentation. The pencil means I can actually take notes and have a PDF record I file under the appropriate client for easy recall.
I also use it, with Duet as a second screen for when I travel, since the 12 inch screen is much more usable than the one on my Air as a monitor.
Keeping the Air2 at a lower price point gives someone who just wants a tablet without the Pro features an option, especially if the lower the price point. It's a lot like the current MB lineup; if all you need is OSX the entry to level models are greater, but if you need more power teh higher end models offer that. It's all about offering models at various price points to entice buyers with the features they need at a price they can afford.
As for developers, more sales of pencil enabled iPads means they are more likely to add features to make their apps appeal to Pro users and a greater base for potential sales.
Apple, meanwhile can add features to iOS to make better use of the Pro features to make Pros more compelling for buyers who want a more powerful OS.
YMMV.