That's the very reason I didn't use any Metro apps on my work computer. They would get swapped out of memory and killed whenever I ALT+TABbed from them. Not joking. Every time I wanted to switch to a normal app for a second and then back to the Metro one, the latter would have to "restore" itself (and guess what, it never restored to the point I last left it).

Of course I know the reason - that's because my machine is memory constrained, I routinely run on ~90% RAM usage. But hey, that 8GB of RAM isn't here for vacation, it's to be used.

Personally, I find the mobile app lifecycle - which is where they took the idea from - to be incredibly annoying. I like to have control both when applications die and when they start up (which, on Android, is utterly unpredictable).