There are many Windows Phone apps, for example Xbox apps, which are not available for many regions, outside United States. There may be cases, when an app is available for all of the regions, but there may be some offer going for some other region, which you would like to avail on your Windows Phone. So, this tutorial of ours shows you, how you can install the apps not yet available for your region or avail offers available in other markets than yours.
While, you may not be able to use the US specific services after installation on your phone, but there are many apps, which don’t require US specific sign-up or which may be on discount in US or any of the regions.
One more important thing is that as you change many regions on your phone, a region specific transaction account gets added to your “Microsoft account” in background. So, it works without any hassles.
So, you are good to go. Try and do let us know!!