Why Does Intra-Party Discipline Matter?

Over at A New Kind of Third Party, TiradeFaction and I are discussing how there needs to a balance between intra-party democracy and discipline in order to maintain a party "brand" and to get things done. The use of party-list proportional representation is a way to increase intra-party discipline. When the party-lists are very short, this wouldn't give party leaders too much power.
dlw