(usually in order to save the battery). Currently the available update periods range from 30 sec to 1 hour. With an hourly period, 3GW runs 24 updates per day. Each update takes around 150 ms (clock time, not CPU), thus ~3.6 seconds per day of processing clock time. This will certainly not drain the battery or bring the CPU to its knees. So there would be no benefit to increase to period beyond 1 hour. Besides, the 3GW widget and notification bar refresh depends on this update, and 1 hour is already a lot.
A final important reason: when the phone is rebooted (turned off/on), it resets its internal counters, and there is a risk that the usage since the last update be lost/not counted (3GW tries to detect this situation and work around it but it’s not guaranteed to work at all times). The higher the update period frequency, the lower the risk, so a high frequency is recommended if accuracy is important.