Well, MMP breaks down when you realize you need to define geographic areas of ridings that you need to lump together so that you can get the ratios right. An example elsewhere in the post points out lumping Victoria’s 4 seats and the rest of Vancouver Island’s 3 seats. If all lumped together, you can get the ratios of actual votes to match the representations of the MPs pretty good - but ultimately someone has to sort of ‘fix it in post.’ If 80% vote for party X in all 7 ridings (which, without looking at the data, I will concede in advanced has never bloody happened) you’re going to take one of those ridings and hand it off to an MP that didn’t win to represent the collective 15-20% that voted the second place party that might be popular there. Which riding gets the MP not elected in the riding? Of course, we need to keep ridings because the population density is very skewed in Canada. If you take a look where people live, you’d realize without ridings, in a true PR setting, the Windsor-Québec City corridor would forever run the rest of Canada. Why try to get votes anywhere else? Do you really want to give Alberta another reason to say that Ottawa has no mandate in their province?

Another option is to drastically increase MPs (that seems like a terrible idea) so that if the riding is 55% for party X and 45% for party Y, you can have 2 MPs from both parties and not add any advantage to anyone to help in forming government. It would almost be a better idea to have a run-off vote until you reach a true majority instead of a plurality in a riding.