Webb told MacRumors that because of the increased spacing between antennas "the antenna diversity will work better" meaning "the probability that one of the two antennas has a better signal is higher."
Ergonomically, the taller phone means the average hand won't be able to cover both antennas. It's harder to hold "wrong". Also, since LTE operates at a lower frequency, 700 MHz, the additional separation is even more valuable.
The aluminum body of the new iPhone 5 will also help the antenna performance, and the hand sensitivity issue will be further diminished. The key word is sensitivity. In other words, changes in performance due to changes in "how you hold it" will be diminished. The antenna will see less change in the "ground mass" of the device.