This is a very interesting point. I had thought the greater freedom of Congressmen in the US to vote as they wished, even if it was against party lines, was strictly more democratic and therefore better, but I'm not sure that's necessarily the case.
For a given issue raised, there are generally several distinct solutions proposed. These proposals have advantages and disadvantages, individually each is probably aligned with the ideology of one of the major parties, but let's say it is not necessarily obvious that one proposal is strictly better than the other. There should be discussion and deliberation, but in the end a well-functioning government should pick one of these proposals and stick with it.
Without strict party discipline a party will have to lobby it's own members and, since it's generally trying to push through multiple proposals at once, choose its battles and compromise. As a consequence, rather than one of the well-thought out original proposals, a compromise combining elements from several is what will eventually be passed. While either proposal individually would work well, such a mixture can easily become an incoherent mess, and strictly worse than either individual original proposal.
With strict party discipline, the party decides in the party room what proposal to advocate, and then it expects party unity in pushing it through parliament.
It's somewhat counter-intuitive, but I think that in many situations it's more important that a coherent and well-considered decision is made than it is that all viewpoints and positions are accomodated.
The wikipedia article on party discipline points out that in Westminster systems, the Executive branch is formed out of the Legislative branch. Consequently, inability to control votes in Parliament literally means you are no longer in government. Most MPs greatly prefer being in government than being in opposition, so there's another inbuilt incentive to vote the party line.
In Australia's Parliament each chamber is elected on a different system. The House of Representatives is single-member electorates. This means that one of the major parties typically forms a majority that can in turn form an executive government able to pass motions of confidence and supply. In short, to govern. Only twice has this failed to occur, leaving the governing party to rely on independents to form their majority.
The Senate, by contrast, is formed from a multi-member scheme with an electorate per state. There are 12 senators per state, half going to the polls at election time. In practice this means the majors capture the 2 of the top 4 spots each, one of the majors captures the 5th spot, but frequently a minor party wins the 6th spot.
This means there's a mix of voices in the Senate. While a lot of legislation gets passed with the agreement of the majors, for anything contentious, the government will need to negotiate with the cross-benches. Right now it's a quite varied bunch.
That and party discipline means that US-style horse-trading is much harder. There's still pork-barrelling, but it's driven by the party's assessment of which seats need to be shored up, rather than every single electorate getting bacon grease, squeaky or not.
Australia has mandatory voting; Hotelling's Law causes the major parties to consistently converge on the median voter.
Voluntary voting radically changes the dynamics. The more apathetic the median voter is, the more powerful the motivated outliers are. The more powerful the outliers are, the more discouraged the median voters become, creating a nasty feedback loop.
If anything, the major reform I'd introduce in the USA would be compulsory voting. It's also the least likely to be introduced, given the prevailing political mythology.
In that sense the USA already has a coalition party structure: various voting blocs form within the two parties and occasionally defect against the rest. In Australia we call them "factions", but their role is to seize control of the party and then enjoy the fruits of party in discipline. In the USA they can instead directly exercise their votes on the floor of Congress.
This is partly a consequence of the ability to mix appropriations with other bills and for amendments to happen in committee. In Australia neither of these is possible: appropriations bills can only deal with appropriations or they become totally invalidated, and amendments have to be moved in the open. Party discipline becomes necessary under such conditions, but the Parliament as an institution is arguably more functional.
Another cause is the primaries system. Members of Congress are answerable firstly to a constituency that represents outliers of outliers, not to the general electorate. They are motivated to occasionally defect.
In any case, even if you introduce multi-member electorates, gerrymandering doesn't vanish. And the USA needs a number of other electoral reforms, too: independent electoral commissioners, replacing first-past-the-post with another counting method and so on.