I'm a new author and I make half my money from KU. I figured it was easier to master the marketing strategy for one platform vs trying to figure out how to get sales from other places. I may go wide when the KU money dries up, but not anytime soon.
C. Gold's post above and mine show why there are no definitive answers when it comes to marketing. I originally was in Select, then decided to go wide using D2D. I now make about half my sales on other platforms (Nook, iBooks, Kobo) while still selling on Amazon. So the best idea is to experiment and see what works for you.
1) You're not really an author if your book isn't on Barnes & Noble, the largest bookstore chain in America.
2) Lots of people refuse to buy from Amazon because they see it as a monster on the lines of Wal-Mart.
3) Kindle is a minor player in many countries, including Canada (where Kobo is the big dog), Scandinavia, Eastern Europe, Africa, and much of Asia including China.
4) Everyone who refuses to play Amazon's exclusivity game is one more roadblock in its effort to monopolize the book business. It launched KIndle with a 35 percent royalty and only went to 70 percent when Apple upset the applecart. You've seen, I'm sure, how the return on KU books has gone from $2 a borrow to less than half a cent per page read, or less than a dollar on an average book.
5) I make more money this way.
(Don't trust KDP to publish a print edition. Don't trust CreateSpace to publish an ebook. Each does one thing well and the other thing poorly.)
For others, it is not. And then again, for still others, it is.
The point is, most writers' mileage varies, depending on everything from genre to reputation to marketing.
I use a mix of tactics myself, in some cases going wide right off the bat, in others going into Select for the first 90 days then coming out and going wide, and sometimes offering a pre-order discount price (but usually not).
Eventually, I go wide with everything. I, too, make the most money that way.
I base my decisions on best guess, a bit of research, and past experience. Recently, I brought out a nonfiction anthology, gave it a pre-order lead to get some ranking, and benefited from KU page reads. Shortly I will bring out a novelette, going wide immediately.
No absolutely right or wrong answer here.
Oh, and yes, do use CreateSpace rather than KDP's print option. Much the better option.