I did not say that vote splitting never happens but to affect a result is extremely rare -- and the myth part is the part where people assume that all the voters of one party would have the same second chocie which we know is not true.
You can't know the effects for sure unless you actually have a ranked-ballot/run-off system. But based on the polls, the ONDP was the most popular second choice of both the Libs and the PCs, in this particular election.
The PCs would probably have gotten the most seats, but not a majority, allowing the NDP to govern with the OLP.
The biggest reason is that there is no reason to have this second choice voting since you can have a proportional legislature -- you cannot have a proportional single leader.
Some people critisize PR for creating Pizza legislatures, with too many single-issue parties and often no clear election winner. If we can't get PR, why not consider a semi-PR system with ranked ballot?