The problem with proportional representation is that it disfranchises entire districts.
This is problematic when representatives are supposed to be working for their districts.
For a US example, it'd be messed up if you were stuck in a district that was mostly democratic but with SOME super hardcore conservatives in it... and say since they weren't going to get their guy in anyway... they all voted for the consitutional party.
So it's something like 65% Democrat, 25% Republican, 10% Constitutional.
Then said district finds out the constitutional party guy is going to be their rep because the Constitutional party got enough votes for at least one rep... and their district had the highest vote for the CP.
Can't imagine that 65% will be happy knowing they voted for the guy who was going to fight forever for new funds for a new park... and they got the "No spending for any reason, in fact lets give money back" guy.
In a way proportional representation leads to NEGATIVE voting for third parties where people would try and "game" the system by voting for extreme parties knowing they will be able to overtop a strong majority simply due to the fact that the seats have to come from somewhere.
Not because they believe in said groups, but because they know that said groups will get a seat somewhere.... AND will work with their party of choice.
