But its not fully implemented
Why we see resume screen then and app crashing
Yes, "background tasks and services" are fully implemented by Windows. What's not complete, is many users' understanding of the concept of "background" as relates to programming for Windows. The term doesn't mean "behind another window" (i.e., in the background of a bunch of open windows), it means "while my task is not running" (i.e., has no windows). [This isn't strictly correct, but it's a lot closer than the former.]
The "background tasks and services" used by an app are special parts of the app that are both separate from the part that users interact with and that are allowed to run when your app itself is not running. It's best if you think of "an app" is being several different program packaged together. One -- and only one -- is for you to use and interact with. The others, if there are any, are there to take advantage of the background services offered by Windows.
"Resuming" is shown during the time between when the OS is ready to switch to an already-in-memory app that's been suspended and the time when the app itself is ready to draw it's screen. Many older apps weren't written to recognize such a reactivation so they instead act as if the app is being started rather than resumed. As such, these older apps do lots of start-up work that isn't necessary, delaying the display of the screen.
This is why users typically see Resuming for longer periods of time with older apps originally written for WP7 -- that platform didn't have support for this kind of task switching and resuming. WP8 apps built from a WP7 base sometimes inherited this unawareness that lots of work could be skipped during a resume; some WP8 apps added a little bit of new code to optimize switching start-up, but the fundamental nature of the app remained.. [As above, this isn't strictly correct or complete, but believing this will get you closer to the truth than where most folks are now.]
Example: I wrote an app for personal use. (It is not in the Store.) I wrote it during the days of Windows Phone 7.0, upgraded it for Mango (7.5), and upgraded again for WP8. I never did a WP8.1 version of it. It has no background tasks. It exhibits the Resuming behavior. I have now rewritten it from the ground up as a Windows 10 Universal app. It still has no background tasks. It does not show Resuming. Because it's built to handle the new way of doing things.
Moral: Microsoft has updated the operating system and now developer need to update the apps to take advantage of the OS.
It's worth noting that this issue is because apps haven't been redeveloped in a while. A lot changed with Windows 8.0 for desktop and 8.1 for phones. Given how difficult it is for firms to justify the cost of writing a WP app in the face of low global market share, few Rewrote the app for the WinPRT API in WP8.1. Windows 10 will hopefully be popular enough to change the market economics so that it's worth writing a new app for phones to gain access to the desktops, laptops, and tablets of the business world.
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