I can tell you, as a lefty who shot righty rifles for years, being able to single load rounds into the chamber with my support hand and visually verify if the chamber is clear/safe was nice. Down side: when you single load a round with your support hand, you still need to work the bolt with your dominant hand. So, its extra hand work. If you're not on that much of a time constraint (like in F Class), then life is good. But, if cumulative time is a factor like in certain competitions (NRL/PRS), then that extra couple seconds of hand work can make the difference on whatever place you finish.
Now, I shoot lefty bolt guns and just keep support hand in place and do everything with my dominant hand. But I mainly do cumulative time events, so thats just me.