Doesn't matter if unlocked version doesn't support LTE band 5. AT&T is not going to suddenly shut off the other bands and leave a few hundred millions of their users lack of LTE access because they don't have a phone that support it.
AT&T's LTE Advanced allow speed faster than 100mbps use the combination of band 4 and 17, the unlocked version supoprt both bands. I'm not sure if a specific modem or hardware is required for this "carrier aggregation" to work on AT&T though. Base on the specs, the Moto X have dual antenna (MIMO for LTE), so it probably should work with LTE Advanced, I'm not 100%. If it is missing, then as long as you don't need 100mbps+ LTE download speed, it's not too bad. But overall, it's a non-issue with lacking of LTE band 5.
Last edited by eksasol; 13th March 2014 at 04:02 AM.