I use Live Id on all 4 of my PC's. You can turn off the Sync function in Sky Drive and the Bing desktop search function. That will kill a lot of the info that goes to Microsoft. I sync some items across all PC's but not all items. My Live ID is used for more than just my Windows Logon. It's tied to my Microsoft MVP Account and MSDN/TechNet subscriptions.
As you said, Live is good if you have more than one Win 8 PC. As pointed out earlier, I like and use SkyDrive as it makes my life easier when working at school on the laptop, and than being able to access those files on my desktop at home without breaking out the laptop. And vice versa from desktop to laptop. Plus it makes Office 365 a lot easier to share and use. Additionally you can have your e-mail sync'd to Windows 8 mailbox.
With the Live account incorporated in 8, it just makes things a little more convenient across the board in my opinion.
BTW I'm wondering how many people have Google accounts or Smartphones? They aren't any more secure or safer than a Live account. But, hey... Live is a "Microsoft" product so...
Meanwhile Google continues to be scrutinized for their personal information collection tactics. And let's not forget about the other social media programs out there.
I'm just saying.