Toss, Yes, of course plays a major role in determining the fate of a match. but as their are many many aspects in a match of cricket or any other sports which contribute to the teams winning probabilities. one can't say in damn sure that the team which has won the toss is going to win the match.
a game is decided by the players performance, their form ,their teamwork and many more aspects like coach of the team, playground where the match is being played. the players fitness, their talents/capability etc.
so what I think is winning a toss to win a match is not of such a hype and important but it should be treated just like other aspects which I described here and other many more hidden aspects.
Even if a single player's form can decide the fate of a match as we are the witness to our beloved "Master blaster Sachin Tendulkar".
Similarly an organised and a great teamwork can also increase the match winning probability to a great height. As we have seen it in Bangladesh and Australia's NatWet series 2005, in which Australia got defeated due to great teamwork of Bangladesh.
So it is just some people's perspective that winning the toss determines the fate of match. winning the toss surely provides the advantage to decide what to chose that is to field or bat first, according to the climate and hence increase the probability of winning the match if both teams are of same caliber and nothing else.