But I just went back to my Office 2007 PPT machine and successfully unchecked the 'show in multiple windows in the taskbar' option that is in the 2007 version and I could see two PPTs open on two monitors - both with ribbons showing. So the functionality to edit two PPTs on two separate monitors was already there in 2007.

Maybe there is just some hidden technical reason why they changed it in 2010 - probably something to do with actually displaying and playing two live presentations would be my guess.