MMP systems encourage greater diversity is political views and a better field with which to express them. MMP systems also lead to a government that better reflects the people it represents. Let's say that you are a Canadian living in Riding X, and that you think that Candidate 1 from Party A is doing a pretty good job of representing your riding in parliament, but you think that Party B's policies are what is best for the country. Under Canada's current "first past the post" system, you are forced to choose between your riding and your country. With MMP, you can make the choice for both.
I do not know if it would promote people I like to win office. I guess it probably depends on the country. But, I can say it is far more democratic. The right principle for democracy is a government in which everyone's vote is worth the same. Non-proportional systems mean a vote in swing districts is worth far more than a vote in a normal district. Therefore this is unfair and should be replaced with a proportional system. Mixed member proportional is a good form of proportional representation. So I support it.
Here in the US, everything is funneled through the two-party system, and unless a new party can get a majority of votes in any single district, then it is doomed to lack any influence. An MMP system allows new ideas and new parties to start up, and gives voters much more choice. Much of the partisan gridlock, voter apathy and ignored issues (why doesn't either party take the NSA wiretapping or campaign finance reform seriously?) can all be traced back to the current system and its limited choices.
This system relies on the voter knowing more about the parties and their 'list'. Instead of having to know what Candidate 1 supports and what Party A supports, you also need to know the line up (who the party will give seats too) for that party. One of the main reasons People don't vote in elections is because they fear they will make the wrong decision and because they feel they don't know enough on how to vote. Presenting the population of the UK, US and Canada with this system will only deter more people from voting.