One of the arguments against PR is that it gives more power to the MPs from fringe parties: for example, the independents and those from single issue parties. This is because the government may need to win over their support to carry a motion or form a government. The fringe thus has a disproportionate amount of power.
But lets be clear, each seat in the House of Commons has the power of one vote, no more, no less. Rather, the influence of the marginals comes about because main party MPs can't act independently, purely in the interests of their Constituencies, because they are governed by the power of the party whips. So, from this perspective, it's not that the fringe parties have more power than they deserve, it is that average MPs have less.Furthermore, with fair electoral boundaries and PR that allows more than one MP per Constituency, each MP is likely to have the support of similar numbers of voters, underlining the fairness of each MPs appointment.
Whether you agree with this analysis or not may depend on your perspective: if you cast your vote for a party to form a government or you cast it to get a local MP representing local interests. It may be impossible to have both!