I got it while I was in college and the last few years have carried it with me wherever I go, even just in the US. It's come in handy a few times, like when I needed a third form of ID to get a Texas driver's license and when I decided to pop over to Canada for the first time while in Detroit. I never thought I'd have to use it to prove my citizenship to another state, however, but word now is that Arizona will be stopping ANYONE and everyone it wants to just to check on their citizenship...because if they only do it to Latinos, they'll get sued for discrimination right and left.
I've driven through Arizona a number of times (on both the 10 and the 40) and there've been occasions where I've had to change planes in Phoenix (flying Southwest), and I've been told by a number of people I come across as more European in nature than American, so I can just see me being hauled off and interrogated unless I have that with me. It's the latest in police state behavior, and basically what was done to Jews in Germany, and the GOP is doing back flips to assure everyone they won't REALLY use it to hurt REAL Americans. But that's bullshit.
Latinos who were born and raised in the US have been deported to Mexico because they couldn't prove their citizenship, immediately. Hell, Cheech wrote a song about it called "Born in East LA" (or was it Chong? I never can remember which is which). It also happened not that long ago in California, where a young man was pulled into Immigration and, because he was mentally disabled and terrified, it was assumed he couldn't speak English, so away he was sent. It took his family weeks to find him and bring him back, and was a huge embarrassment to Immigration officials. Now? It'll be shrugged of with, "We's just followin' the law."
So if you're going to see the Grand Canyon, take your passport. If you don't have one, get one. You'll need it in Phoenix, Tuscon, Sedona, Flagstaff and any stretch of freeway you're heading down. Hell, soon you'll need it in all 50 states, if the GOP has its way.
Right now, I'm wishing I'd gone and started teaching English in Berlin. I thought about doing that when I visited friends over New Year's 2007/2008. But I wimped out and focused on working in film as a writer...and look at me now. Working in Buffalo.
Moral? If you think you want to do it, don't think it -- DO it. Nine times out of ten your initial instincts are the truest and best.