Parliamentary systems have nothing to do with having a two-party or n-party system. Parliament is where the people go after they've been voted in. having 6% of the people vote in 6% of the members of parliament is about the voting system used. The one we use (and the one that you use as well; I'm Canadian) is First Past the Post. FPP has the advantages of being dead simple, there is a direct link between voters in an area and the person they elect, and it tends to favor majority governments (which then have the ability to get things done for the length of their term).
What you're asking for is a proportional representation system. The downside of such a system is that you lose the geographic link between the voter and their representative. In a small country such as Israel this might not be such a big deal, but in as geographically and culturally diverse a country as America it probably would be.
There is also the fact that coalition governments aren't as stable as majority ones, which means that there would be more elections, which most people probably don't see as a desired outcome.
As far as we can tell, the theoretically best voting system is probably the Single Transferable Vote system used in Ireland, Malta and some other places. But it's complex and would require either way more elected representatives, much larger voting districts or some combination of the two. I'm not going to even try to explain STV. There must be a decent wikipedia article on it.
You could use any of these voting systems (or any other one, even randomly electing people) to elect your representatives and still have a parliamentary system.