For a start, if you don't know what STV is then check this out for a fairly good explaination.
This is a voting system that is called proportional...though the caveat is that you don't get power for a small percentage of the votes, and thus the process means it isn't EXACTLY proportional. This is a good thing as it ensures that flash in the pan small issue politics don't drastically affect the make up and alliances of parliament.
With this in mind the key reason that we should support STV is it is stable. While it is easy to change who you have as your representatives the "swing" in change is unlikely to be huge. Take 1997. The swing which saw Labour shares rise 9% while Tory shares dropped 11% would have (on today's seat numbers) seen a change of roughly 70 Tory MPs losing their seats while Labour would have gained just under 60. A significant change, but a long way short of the Tories losing 171 seats and Labour gaining 147.
The sheer level of change under the 1997 election meant priorities were completely rewritten. There was no transition, there was no balancing factor of a third party being needed to support the main party, there was no stability in how our country was governed, no consistency.
The other thing that STV brings to the table through this is parties needing to set out their own niche. At the moment the parties don't need to worry about large swathes of the seats in the UK. They are safe, staunch support from tribal loyalty means that barely any money is spent on them. The key seats are the few marginals, where there are the undecided swing voters.
Swing voters are profiled, they're examined, they're brought in to focus groups. All of the three main parties do this, and it results in one thing...convergence.
The parties are not all the same, but their messages, the shape of their key policies, they're all extremely similar. This is because they need to win the support of just this minority of the electorate. STV breaks that apart. EVERY vote matters, EVERY vote could change the shape of power. Suddenly the emphasis has to not only be on what the undecideds want in a few key seats but what ALL undecideds want, while having to seriously balance that with the wishes of their more faithful support.
If we want our parties to talk together more, to work out how to represent all of our views best, we need STV. If we want to have a better choice between different parties with different policies, we need STV. If we want our votes to mean something wherever we are in the country, we need STV.
If you agree to make sure in a referendum that we talk about a choice between FPTP or STV, then please sign this petition.