It's all about how the votes are distributed. Each constituency has chosen a party/mp to represent it; the people in that constiuency want to represented by that party/constuency. This, in turn, is how we choose the overall representative of our country.
Now, as far as i can see, if we went purely on P.R (the total votes) then our representative party would be chosen more by the extreme voting areas of our country. Those areas which contain more voters.
This could cause a problem with the rising popularity of BNP up north. If the votes not the constiuencies down south were looked at, i bet, due to the voting inertia in the south, that BNP would become much more controlling of our country.
I can think of many reasons pro-P.R but i'd be interested to see what things other people come up with.