"Most important companion since the previous most important companion" refers to the usage of a companion as a walking talking plot device. Instead of character building, we get deus ex machinas which everybody loves.
The classic series had a Big Plan for Ace, but the writers never pulled it off. Then there was Cwej, Ace again, Romana's daughter, Sam, Romana 3, Compassion, Romana 2, Miranda, Charley, etc., but nobody watched the webcasts, read the novels, or listened to the audios.
So it was left to NuWho to really run with this idea. Starting with Rowse Tylah, there has been a long and proud tradition of most important companion since the previous most important companion. Most recently, Pancakes, Danny Splink and The Brigadier have held this title. Each time the showruiner attempts to top the previous companion.
List of most important companions since the previous most important companion: