Concurrent voice/data is a HUGE feature.
Just like I wouldn't buy a car without auto-transmission or power windows, I would not go to a carrier without concurrent voice/data. Once you use it, you rely upon it.
That's crap. I've had the AT&T iPhone, too. Is concurrent voice/date nice? Yes. Is it "essential"? No. Consistent, reliable 3G coverage is a helluva lot more essential than concurrent voice/data.