SE net sometimes gets used for serious incidents that require a separate dispatcher and channel due to the volume of radio traffic, and length of time interfering with the routine radio traffic in that precinct. (The shooting of fugitive Kelly Swoboda is a recent example of one such incident.) It's also sometimes used for special detail events such as Peacock Lane during Christmas time.
Personally I haven't heard NE net get any use since the consolidation of the former 5 precincts into the current 3.