PR is not necessarily bad. It moves towards consensus politics but that relies on the Govt being able to reach a consensus. When they can't nothing gets done.
Yes, at the moment a Green vote is a Green vote but in many places a Green vote is a wasted vote. PR could change that.
People are sometimes forced to vote tactically to keep the bad option out, which leaves them with the better of a bad lot but at least they know what they're getting and they know that it (probably) won't be sold on to form a coalition with a party they didn't vote for.
PR works when you have a number of parties who are roughly the same in terms of political power and then a few smaller ones who can be represented. If the Greens became a real political power we could do it but at the moment PR means we're likely to end up with exactly the same result as a coalition formed from FPTP. 1 lone MP isn't going to be able to push a Green agenda effectively. PR requires a certain critical mass to allow voting blocks to form.
One way of doing PR is to create super-constituencies by merging existing ones and creating, say, 5 seats per super-constituency. Then each party would put up, say, 5 candidates each and operating a single transferrable vote knocking candidates out until it's the last 5 standing. Then it would depend how high people rank the Green agenda whether the Green party got any power. Unfortunately this does let in the BNP. Greens clearly wouldn't end up with a majority in Parliament but they would have more of a block and be a force to be reckoned with. In theory. They would have more input, there would be more debate, the Green agenda would need to be taken into account and it would raise their profile. The flipside is that we couldn't put a whip on to pass legislation, things would be much slower, coalition building would take longer and then there's the architectural factor in the house of Commons!