Actually, I should point out that I have been using 5 monitors since Win 98 was released and I realized that the IT intern who sat next to me had a pile of disused monitors and PCI video cards he had pulled out of PCs that he was upgrading.
I do occasionally find a reason to have a window span multiple monitors but on those rare occasions I don't mind manually resizing the window. I would have to say that Microsoft made the correct choice in this case.
And for what it is worth Mc and kinokijf, since my bottom right monitor is Primary, it means that 4 out of my 5 monitors have negative coordinates. I see instantly when a program is poorly behaved in those circumstances. Over the years programs have gotten a lot better. In the VB5/6 days most programs had huge issues with it. Nowadays most do fine, with the occasional menu or tooltip issue being the most common. Oddly I see a lot of programs that seem perfectly OK with negative X coordinates, but still freak out about negative Y ones.