The problem with most forms of PR I've seen suggested (including the version we voted on in 2011) is that it breaks the link between MP and constituents.
A good MP really gets to understand their constituency, and tries to represent their interests, which should mean that no area - or their specific concerns - are forgotten just because they're far from Westminster and not considered important.
MPs are already too tempted to think of themselves as the elite, and focus on power plays in Westminster, forgetting that they are meant to be representing us. Think how much worse that would be if they weren't tied to a constituency and could just airily imagine themselves the rulers of the UK.