The key reason to use WP for me atleast is
1. Stability
2.Performance over time without me having to stream line things, (all droids get slower over time even the legendary nexuses)
3.This is more of a personal thing but i like the UI WP 8/8.1 has(still havnt tried WP10 ) I dont think either droid or Iphone have this kind of at an glance UI that gives access to info as well as apps on the go! Ubuntu phones are the closest to WP in terms of an interactive UI but they are still far behind
4.Gives me a unique option other than the common droids and iphones
5.The APPS i need are mostly available through official or third party ones.. but this is most likely to always be the Achilles heel for WP
If any of the above mentioned is going to be affected in the coming WP10 it might be an deal breaker for me, i use WP because to borrow apple quote it just works and hardly requires any maintenance! Its a great replacement for BB phones.
As long as WP10 doesn't affect performance, as well as stable UI without glitches i would be more than glad to get one without worrying to much about app ecosystem!
Hope they get it right! Cant say im really happy with MS as they seem to be jumping from one fiasco to other SB launch glitch, one-drive storage reduction etc.! While some may say onedrive is hardly related to WP, but its MS as a whole we want to trust and invest in more than just the product so it matters!