The Australian system is certainly not perfect, but I think the indirection of the "leader of the party" choice and Prime Minister is helpful - it promotes a greater willingness of the party members to say "Okay, you're hurting -my- re-election chances now", with the result being, whether the ultimate motivation is self-interest, you often (or much more when I was younger) would see no-confidence votes and leadership changes, certainly far more than in the US, where I am now. And signals like you mention, "explicit non-support" seem to carry greater weight.