The idea
Parties should not be allowed to change leaders midterm without seeking a new mandate from the public for that the new leader within a very short period of time. Whilst we don't have a presendential system, it is obvious given the prominent position of the leaders in the election campaigns e.g. their role in the leaders debates on TV, that people are still voting the national leader as much as the party they wish to govern. If we are not changing to a presendential system then the next best thing is to remove the power of small cliques of MPs to change who the Prime Minister is. The democratic contempt of small groups of MPs and cabinet minister shown recently with their plots and vested interests deciding who the PM is a huge insult to the electorate. The PM sets the course of the nation and can take this country to war, the post should not be the whim of a few party insiders to decide.
In recent years we have had two unelected PMs (Major and Brown), both imposed on the nation by coupe d'etat. In the last parliament the man who won the mandate (Blaire) said in plain english he would stay for the whole term, i.e. no "vote Blair get Brown". What happened ? The exact opposite. During the recent election we were only a few seats away from having three mandateless PMs back to back - Brown (steps down to enable LibLab pact), Harmen then as stands in a temporary leader (as she is doing now) then finally a new Labour leader (Milliband, etc) emerges as PM months after the general election once Labour's internal political process had played out. How can we talk to the world about democracy with a system like that ?
Why is it important?
The PMs role is the most important job in the country, in a democrary it should not be the play thing of cliques of MPs. We should never again be governed by someone who has not faced the public as the leader of their party.