Its not about the added leverage, which really is about the same - upper hand is in the same fulcrum position after all.
It does slow the cast down a bit, well the feel of it encourages you to take a bit more time.
Reality for the fishing caster is that there is very little difference.
You can convert to casting reel down with a good 4 hours of practice, but dont expect it to magically add yards to your cast - it wont.
It is slightly more awkward retrieving, but you soon get used to it, also when fishing hardcore at the likes of Rock Channel your reel will be in the water
I do prefer reel down, partly because one of my reel seats has got stuck in that position so i'm now stuck with it!
Make a difference - take home 1 piece of someone's angling litter next time you are out fishing.