This will be debated until the end of time...but besides being easier to adjust cutting height, there is the 'theory' that a floating deck will scalp less, or perhaps less often. My take on the rhetoric around here is that larger cutting decks will benefit from being 'floating' beginning around 48" and larger. Let the debating begin!
But really, as others have said the easy cutting height adjustment is a big plus for floating decks and less scalping...but if you know what you're doing you can avoid scalping with a fixed deck, especially 48 inches and under.
Unless the anti-scalp wheels are set properly you will still scalp with a floating. I wish I had gotten a floating deck over my fixed, but only because of ease of height change. I do not really scalp at all with my fixed walkbehind.