Then you will start to get tons of apps that do not need to run on background running in background and then not working properly on mobile devices.
It's not so much a case of running in the background doing nothing. Lots of apps, desktop apps, just sit there, the only impact they have to the system is that they take up RAM, just like metro apps do. Out of all the apps I have running right now on my desktop only 3-4 are actually using CPU cycles, others sit and wait till I switch to them and do something. I don't see this as a huge impact, even on mobile, to battery life or even performance yet what you do get is faster switching between apps and so on like you do right now under Win32.
Besides, the API request isn't for generic "background tasks" it's for them to raise the arbitrary limits they have in place that can get in the way. Why, for example, do we have to limit the number of apps that can "run under the lock screen" to 8 or 10? Everyone loves choice, so why not just let me pick as many as I want? It's my device and my battery and if we're on the desktop why do I even care about the impact to power? The system should be smart enough to not suspend apps in this case, and it's the OS that handles suspending apps, not the apps, the only thing apps have to do is make sure they save their state when the OS tosses them a notice saying they're going to get suspended or killed soon, that's it.
This is more about the limits of what they have in place now and not the lack of it altogether.