True Full screen is pretty much a requirement for multi GPU builds (ok sometimes it works WITHOUT but this is a 20% chance), and not to mention a game running in full screen has better performance than windowed: even if it's covering your screen size (fake full screen), Windows DOES redraw windows behind it.

The issue is that their DirectX implementation fails, and loses the "state" of the process when Alt-tabbing while in full screen mode.

Why this is an issue, will never be known as SE are the only ones capable of such retarded bugs.