Flukey, the main arguments are that it's easy for an already disenfrachised electorate to understnad, and it provides strong government - you're far more likely to get a hung parliament in a proportional representation system (this is where the party with the most seats still has less than everyone else put together) and this can give undue power to small parties, becuase they may get what they want through compromise with others in a coalition.
For example, imagine you have a PR system where Party A gets 45%, Party B gets 40% and Party C gets 15% - Also, imagine that one of Party C's policy pledges is to reinstate capital punishment, where as Party A and B don't want this. However, Party B and C could form a coalition to gain government - as that would give them a 55% majority - but at the expense of them having to allow capital punishment, if Party C demand it.
It's like the swing states in the US - a little percentage can give the little parties enough to be able to dramatically alter policy of the majority parties. While this is entirely possible in FPTP, it's far less likely.