This is a big benefit to those that travel overseas quite a bit and want to pop in a SIM card from a local carrier instead of incurring those large roaming charges. Voice roaming charges are high enough but once they start dinging you for data and text on a trip, you could start a nice college fund with the roaming charges you'd pay.
If you have left T-Mobile for some reason and want to continue to use your phone on another GSM carrier like AT&T you'll also be able to just use your AT&T SIM. The only problem with that is you won't get much more than voice, text and EDGE data services. T-Mobile phones and AT&T networks don't mix when it comes to 3G.
These changes also apply to business and government accounts, so if you are responsible for roaming employees, this may be a way to lower your telecom costs.