The three rows where for the larger screens only ( i believe 5"and up ).
But due multiple requests of many users they made it possible to enable that option in WP 8.1 also for the smaller screens.
So for the smaller screens you have a setting to use two or three rows.
The larger phones dont have that setting and are "stucked" with the three rows.
For the visual aspect.......( my opinion ) three rows on a 4" phone looks cool at first, but the smal tiles are to small.
Other way arround, i think that if you had 2 rows on a 6" phone these tiles are way to big.