As a Dane, I'm a huge fan of the proportional representation established here. The major reason is that the division of the triple power was only in place during our constitution's actual implementation. The judicary branch is as it has always been, and the legislative branch was elected proportionally. However the king was intended to be the executive branch. Today, Danish royalty's signing of laws is purely ritualistic. Parliamentarism with no proportional representation would procure systemic issues; then the sole parliamentary power would elect the sole executive power, and that's the path to corruption. As is, the government sometimes comes together to execute some horrible laws (as when pensions were cut, they didn't cut pensions for the politicians themselves), but it's quite rare, and I attribute that to the fact that parties are so dependent upon each other's votes and support.
Of course, I don't know enough about systems outside Denmark that have made parliamentarism work without proportional representation. No, the pseudoparliamentarist properties of the US don't count, because their governmental structure really isn't triple, moreso sixtuple, if that's a word.