The sky is falling.
If you (or microsoft) thing this is going to make the tiniest difference they're kidding themselves. Maybe some kiosks might use it.
All you have to do is consider how many iphones would be jailbroken if the only impediment to doing so was finding a tickbox buries 3 or 4 dialog boxes deep.
Grandmas and grandads, and those who use laptops (and even desktops) for basic productivity apps and wouldn't know what a linux was if it jumped up and bit them will benefit - they'll be in a nice comfortable walled garden where the only apps presented to them have been pre-scanned for vulnerabilities, and where the nice people at Microsoft will keep them far away from dodgy apps and anything that smells slightly of piracy (even if it can be used for other purposes). All their apps will either be metro interface or web hosted. and everyone's life will be happier, because the likes of you and I won't be having to drop everything every two weeks to go over to and marge's and delouse the computer because of the latest email she received that 'looked like it came from her bank'
Anyone with any interest in Windows PCs as technology for creative purposes, whether that be coding, cad, photoshop or anything else not available (or severely hamstrung) on the store will tick the little box and be able to use full windows in all it's former glory, and with all the usual caveats. Sure, you'll also be able to run linux if you so wish, or run it in a hyper-v vm or whatever floats your goat.
As for the conspiracy theory that Microsoft will eventually lock the gate of their walled garden, I can't see it as basically that means anyone with any acumen at all will be a die hard linux guru - and those are the guys that will eventually be deciding data centre policy in large corporations and development target systems in software houses. Locking those guys out (or in) really is shooting yourself in the foot.