This statistics represents shipment, but doesn't mean that the retention of a windows phone might have been higher (maybe the one who had a Samsung s4, wanted to go for a S5 this year, and maybe the one who had a high end Nokia Lumia didn't need the change).
However, 2.5% worldwide its a lot, its millions of users! I know its 40 times less than Android, but again there are a lot of developers that thinks "meh, I won't go for the windows phone market because its too small", so the competition in this market is also lower. What is more, Microsoft is a big boy and won't let Windows Phone die any soon. The Windows 10 release will be a big change, you will be able to develop for both desktop and mobile and go to a single Market, so you will hit the Desktop Store as well.
And keep in mind, that if one day you want to sell some app to a client, you will get more reputation if you can provide a complete mobile solution: Android, WP, and IOS (let assume that blackberry its really going down soon
). You will be able to program in one language (visual basic), and prove a complete native solution for all platforms!