said by en102 :

I _really_ don't think anyone's using omnidirectional for data communications. Its a waste of spectrum.
The main reason for trimming to a few miles vs. ~30miles is capacity. How many users can use WiMAX in any given area can use it, if you're going to spread a 70MHz beam over 30 miles, even in one sector. Backhauling is different direct point-to-point, with a narrow area would obviously give a much longer range.