The one thing to remember is that a single MP is not going to be able to represent everyone in that district, because there are many different opinions in a district. To represent multiple points of view, you really need multiple MPs. In that circumstance, you have to figure out a way to elect them.
In a multi-member district, different parties will be putting forward probably one more candidate than they expect will win, because if the election goes their way they'll want to be able to take advantage of that. That gives voters a very interesting opportunity to have a say in the candidate selection process of the parties—all voters—because with the preferential ballot, they'll be able to rank the different candidates in the multi-member district.
It's great for voters. It may be harder for MPs, but maybe not. For voters, you can have a say in it. If there is an MP who is doing a lousy job or who has done something that people don't like, voters can say that they will not rank them at all and they'll rank someone else, so that's very good.
You can keep people accountable if you have an MP who you voted for, but if you didn't vote for your MP and they're running again, how do you hold that MP accountable? You can't withhold the vote you never gave them in the first place, but in a multi-member district where more voters have a representative who they voted for, they can hold them accountable.