First of all - STV offers people the freedom to vote their conscience. Instead of having to choose between a candidate you support and the front-runner you hate the least, you just rank the candidates in your order of preference, without worrying about splitting the vote and making it easier for the front-runner you hate the most to get elected.
If you think that the latter only concerns minor party supporters - think again. There are more than enough constituencies where either a Liberal or a Conservative candidate doesn't happen to be among the two front-runners. Conservatives face the same tough choice in Quebec and in many urban ridings, where the other front-runner is NDP. Similarly, Liberals in the Western provinces (apart from Vancouver,) are in the same situation, since the race is mainly between the Conservatives and the NDP. According to the Nanos poll 64% of Canadian voters have a second choice. Preferential voting, which is used in the STV, allows voters to mark that choice on a ballot.
Another great advantage of the STV is intra-party competition. In other words, Liberals competing with other Liberals, Conservative - with other Conservatives etc. Currently we have that only during the nomination contest, which is, on one end, restricted to a select group of supporters and, on the other end, the results are often distorted, as almost every party allows would be candidates to sign-up hundreds of one-time members to outnumber their rivals. Not to mention all those "star candidates" that could be "parachuted" into a riding, bypassing the democratic nomination process altogether. STV addresses those problems, giving a lot more say to the voters.
Obviously, preferential voting allows anyone who believes that his nomination was rigged or hijacked, to run as independent without splitting the vote. But that's not all. Since STV uses multi-member constituencies, there will be more than one candidate for each major party. Thus - voters could choose not just the party they support, but also - what candidates from that party they'd rather see elected. So, if one out of 3 candidates for a specific party is a "hijacker" who won the nomination through mass sign-ups or if he's a "star candidate", appointed by the party executives - voters have the opportunity not to vote for him while still supporting the party of their choice.
For example, voters can put either candidate B or candidate C as their first choice, then candidate C or candidate B respectively - as their second choice; then they could either stop there or rank an independent candidate or a candidate from another party - as their third choice, leaving the unwanted candidate A distant fourth (or fifth if not tenth) or not ranking him at all. Voters also have the opportunity to split their allegiance, ranking an independent candidate or a candidate from another party as their second choice, then returning to their preferred party and ranking the remaining candidates as their third, fourth and subsequent choices. The options are numerous and the final say is always up to the voters.
This turns out to be an advantage of its own. By being able to pick up specific candidates, rather than being compelled to vote for the only one that's on the ballot, the voters become able to choose the direction in which their preferred party should move. Red Tories or SoCons? Bob Rae Liberals or Michael Ignatieff Liberals? Run both and see which candidates receive more 1s and 2s next to their names. That will send a comprehensive message to the party executives and convention delegates, won't it? (Yes, I know that BC STV would only apply on a provincial level, but it's easier for me to use Federal parties as an example.)
Finally - STV allows us to maintain all the advantages of the existing electoral system. While it reduces the distortion between popular vote and the actual seat distribution, this is achieved without using party lists, with all the candidates (not just 60 or 70 percent) running locally, with each candidate being personally accountable to the voters in his constituency. It does not need an arbitrarily set qualifying threshold and "fractional" seats (if any) are distributed based on users' second and third choices, rather than using mathematical formulas. STV empowers voters, maintains local representation, holds elected representatives accountable while making the final seat count more proportional. That's why I support it.