I can do either, but prefer to reel with my left and cast with my right. It's like a baseball glove on your left hand and throwing with your right hand or vice versa if you're a southpaw.
Those that pitch with their right and reel with their right (doesn't apply to trollers) are very inefficient fishers. It's only a second or two wasted switching back and forth between casts, but throw 1,000 times a day and that's about 17 less minutes of fishing per day. Maybe you're pitching a jig or countdown and you need a few seconds of drop time and that doesn't eat up the strike zone clock as much. But you may be late in a hookset during the switcheroo and miss a bite.
Same thing for a tiller guy who drives the boat and operates the rod with a left hand; it's inefficient passing the rods back and forth.