The biggest motivation I can see for moving to PR is to help out the smaller parties. The NDP regularly gets much more of the popular vote than its perecentage of MPs indicates. Little parties like the Green party get a certain amount of votes, but spread too thin to elect anyone. It's a no-brainer that those little parties would be pushing for a system that benefits them.
But we have to realize that it would not only be the NDP and Green party that would benefit. All sorts of extremist and weirdo organizations would start to elect MPs. Think back to the 1993 election. The Natural Law party fielded 231 candidates, spent millions on advertising, and garnered 84,743 votes. Under PR, they might have elected an MP. Their platform? Kinda hard to say. Their main schtick was "yogick flying" - essentially, Doug Henning sat in the lotus position and hopped, with photographs taken as his bum was a couple of inches off the floor, and they claimed he could fly.
PR does not help Liberals, and it does not help the country.
Here's a recap of the six things I could think of that are wrong with PR:
1. More elections. Lots more elections. This will be very expensive, and it can lead to voter apathy.
2. Strange coalitions. At the federal level, we could very well be governed by a coalition including a Quebec separatist party. In other countries, it's not uncommon for extremes of the left and right to join up against the centrist party.
3. The election of fringe party MPs.
4. Loss of local focus. Instead of our current small ridings where we know the issues and can meet the candidates, we will be voting in much larger ridings and have much less understanding of the candidates or the issues. Depending on what form of PR is in place, there may not even be the concept of a local representative.
5. More instability in governance. With the creation of temporary alliances to form coalition governments, there will be greater swings in policy approach. It will be more difficult for the government to have a strong vision.
6. Undemocratic. Coalitions are created between members of the political elite, without any input from voters. Nobody voted for the coalition. A party that got few votes can wield a disproportionate amount of power.