Are you talking about McHale vs Fantino? Interesting how that was settled. The crown invoked its right to take over the case, then it stayed the charge.I think a private prosecution would be useful (and a caveat: I know nothing about them) in the case where, say, a police officer did something and it got kind of hushed up or dropped for some political reason. At the very least the atttmept would certainly get some press.
I imagine that the laws against malicious prosecution could be used against anybody who really tried to abuse this avenue. Would the plaintiff be on the hook for costs, as well?