Alright a lefty is gonna give you his take on it....
I am left handed, and whenever I fished with spinning reels, I always have switched the handle to the right. My dominant hand has always held the rod, and the right hand has reeled. Fortunately for me, the Zebco's came that way already. Now why on earth do "lefty" baitcasters come with the handle on the left when most fishermen I know use their dominant hand to hold the rod? I don't think its cause you need power when reeling when you're using a casting reel (unless you're on a deep sea trip in Florida with a demented captain and first mate that insist on you using your reel, not your ROD to lift up big grouper from the rocks - the result? Grown men who look like they're having seizures and convulsions trying to reel up a grouper, that most often, schooled them and went back in the rocks, however, I digress....) What was I talking about again? Oh, right, I think the reason is because of tradition. For some reason (maybe bc they followed the pattern of big trolling reels when they started making regular casting reels) they've always been making them this way, and guys have learned to use them this way. I buy "righty" casting reels, and they're perfect, I thumb the spool with the left hand, and then the lure hits the water, I'm ready to roll.
So for once in the right handed world, we lefty's have gotten a break, and finally the right handed people have gotten their recognition, too bad they just don't know it!!
Have a great day