You ask if multiparty systems are necessarily more representative than the two party system of the United States. The answer is not necessarily. First, let us look at an example of how they can be. Picture for instance a government in which the parliament consists of 100 members. If in this government, each voter votes for one candidate and the 100 candidates who received the most votes are seated, then as many as 100 different views could theoretically be represented in the parliament. While it may be that many voters will vote for one popular candidate, it is also possible, and will usually be the case, that some very small minority parties will receive enough votes to send a candidate to the parliament. If the voters must choose between only two parties, and if the election is at large, that is, nation-wide, then only one party will receive a majority of votes, thus only the majority will be represented in the government. If the two-pary elections are by single representative districts, as in the USA, then some districts will return one party to parliament (congress), and some districts will return the other party, so that in parliament, the majority and the largest minority are both represented, but only those two.
Now let us look at an example of how they may not be. Suppose that each voter is the country is asked to vote for one party instead of one candidate, and instead of the 100 candidates which receive the most votes being seated in parliament, the one party which received the most votes sends a full slate of members to parliament from its party only. This would be less representative than USA's two-party system.
This is going to be subject to much in way of discussion and debate. I would suggest that multi- party systems appeal to different niches in the political discourse. In this way, more voices can be heard. However, the more that voice is spread out over a heterogeneous democracy, it seems that this voice becomes more diluted, unable to be heard of any significance in terms of constructing policy and substantive legislation. This is where alliances between different parties are formed by the leaders of these parties. They do so in the form of deals being struck. At this point, one has to determine whether or not one's voice is more resonant than with the two party system, whereby one knows that what is being voted upon would be, for the most part, recognized. Additionally, it should be noted that the two party system contains multiple elements within each party where individual voice can be validated. There are stronger, more ideological factions of each party within it that could also be used to represent individual political temperaments.
Not only are they more representative, but they are more so by definition. Our two party system emerged in the 1790s, when there was fewer than 4 million people living in the entire United States. We are now a nation of over 300 million, covering 3.5 million square miles of territory. It becomes impossible with a nation this size, with a population this diverse, for two parties to even partially represent everyone.
What about those that are almost always under represented? Ethnic minorities, women, homosexuals, political and religious minorities have very little say in our government, and have been easily discriminated against, and some still are. Multiparty systems give them a place at the table, and force the larger, more traditional parties to work with these small ones to form coalitions.
At this point it is almost impossible to define what a Democrat or a Republican is, because the groups are so large they encompass both liberal and conservative members. Not so with a multiparty system.
For the most part, I would say that they are more representative, but I would not say that they are necessarily so.
They are generally more representative because in those systems, smaller parties can more accurately represent the wishes of their members. For example, if we had a "Tea Party Party" in the US, it could accurately represent its members' attitudes without having to compromise.
However, in such systems, parties might have a harder time representing people who feel that there are many important issues. For example, if we had an anti-abortion party and an anti-tax party and an anti-environmentalism party, what representation would there be for people who think all of those are equally important? Or for those who think that two of the three are but one is not?
Voters all have different mixes of issue attitudes and issue salience and even multiparty systems cannot accurately represent everyone.
One other issue to consider is that, in a multi-party system, you are often voting for slates of candidates rather than being able to specifically vote for a given person. You could say this is less representative as well.