While this would be nice, not sure how high it is on the Visible plate. This can become expensive for carriers and is easily abused. That's why most carriers have a clause saying over 50% of your usage can't be roaming.
I'm not saying they aren't working on it, and it will never happen, but for $25 (I'd imagine the majority of their customers pay only this amount), can't expect many of these premium type features
I don't really care that much about international roaming. If I am traveling abroad, I will just pick up a local SIM if I need it. However, domestic roaming is something I care about. I like to take road trips and sometimes while trekking across the prairie one network or the other might not be available for part of the trip. Having the reassurance of knowing I can roam on AT&T or T-Mobile for a bit does make me feel better. That said, I haven't tested Visible on a trip to see if it fails me. We shall see.