Setting aside discussions about valuations, here are a few reasons why Google and YouTube would create an entity greater than its parts:
What does Google offer YouTube?
Technical chops to automatically identify when copywrighted content is being displayed
Negotiating clout when seeking an amicable deal to share revenues off user posted copywrighted material
What does YouTube offer Google?
Search queries – particularly video search queries
A sea of content for Google to syndicate to relevant places interspersed with high CPM video advertising