IT'S BETTER OVERALL to boot to metro, I shall explain why. Users probably boot to get on the internet over 95% of the time, booting to metro is a no click operation to be surfing the net, for instance my pc boots in 10 seconds to metro and I press "enter" and I'm on the internet. And IE closes to desktop when I'm done surfing, You can't do that booting to desktop, you have to move the mouse, find IE and click.
So, as you can see w8 is 2x more productive than w7.
MSFt is trying its best to use Win 8 to gain traction in tablest and smartphones. Once a reasonable goal is reached, MSft will most likely issue a Win 8 Classi or Windows 8 Desktop, that will have the start buttom as well as the Start Screen on the top.
The key for MSFt now is to have all OEMs to aggrassivly push WIn 8 hybrids and smartphones, even at a slight drop in desktops as the price to pay. The good news is, WIn 8 tablest are sellign really well (check Expansys), and WIn 8 Phones are finally selling more than 10 millions in a single quater. WIndows hybrids and smartpheones are now coverring from very low end to very high end, and they are in the right positon to compette effectively from now on.