Why doesn't ATC use a unified frequency for the same control area? If you have experience of getting flight following or flying instrument in the US, you might have noticed that approach and center controllers alternate between two or three frequencies for the same control area. For example, I get 118.1 from the tower for flight following one day, and the next day I get 126.0 for flight following for exactly the same route of cross country. 

Why don't they just use one frequency for the same control area? Wouldn't that make their job much easier?
This situation is due to usually 2+ sectors being worked. Each sector will use its own frequency when split. When these sectors get combined, one of 2 things will happen. 1) The controller will just work with both sectors frequencies keyed up and be listening on both and transmitting on both and have everyone go to the approrpriate frequency for the sector they're in or near. 2) The controller wants everyone on one frequency and has everyone going into those sector's that they are working to be on just one frequency. Frequency congestion and capability(if the transmitters/receivers are different locations for the different frequencies) are also factors.