I basically concur with most of your argument on the grounds that it is the class structure (i.e. distribution and control of material resources) that matters, not th epolitical rhetoric.
I think, however, there substantive difference between the "winner takes all" system like one in the US, and a multi-party parliamentary system based on proportional representation. Here are some of them:
1. The winner takes all system is more prone to avoidance of political issues in favor of negative campaigning than a multi party system. It is so, because in a two-party system undermining the oppostion can directly benefit th eother party, because the voters have nowhere else to go. That does not hold for a multi-party system. Consequently, the avoidance of any susbtantive issue seems like rational strategy to avoid substantive issues that may form the basis for "liberal-baiting."
This is, of course not to say tha tcampaign rhetoric is all that significant, but it is easier to to demand that politicians fulfill their campaign promises than demanding something that they never promised.
2. Proportional system makes partiers more responsive to minority concerns. It is so, because even a small number of votes transates into a political gain - a greater number of seats in the parliament.
Under the winner takes all system - catering to minority interests does not produce any tangible benefits in terms of votes (unless a small block vote can tip th ebalance over 50%), but can be used as a basis for negative
campaigning. Hence the winner takes all system creates a clear disincentive for political parties to address minority concerns.
3. The multi-party system seems less corruptible by the dominant economic interests. It seems easier to buy two political parties than to buy many. Moreover, under proportional representation, party sellouts may cause voters to "vote with their feet" - and that may have a direct consequence for th enumber of seats held by that party. Therefore, parties have an incentive to resist corruption, at least to some degree. Under the winner takes all syste, no such incentive exist, because evef if a small fraction of the population bother to vote, the electoral winner will "represent" the entire population.
Wojtek