If a person has already purchased a top of the line transmitter and paid say $500 for it then flight packs are a reasonable way to get another model in the air.

Using just one transmitter allows total familiarity with the controls and their location which permits operation without looking at the transmitter while in flight.

I personally do not buy flight packs preferring instead to buy the components separately. This allows me to select just the servo (metal gears) and battery (nimh) I want to use.