Nobody's mentioned the other good thing about the split from a business standpoint - it allows them to tour around essentially with two separate rosters.
With the split, they can have both RAW and Smackdown putting on two different shows with "main event characters" in two different cities - at the same time. They can have RAW one night and Smackdown the next without requiring everybody to be there, essentially giving the rosters more time off.
It also allows them to give more TV time to more people. Granted, it's a lot of the same people most of the time, but imagine seeing the same 10 people on both shows combined, rather than the same 10 on RAW and a different 10 on Smackdown (for a combined total of 20 - that's definitely more people) - less of a chance to tire out your viewing audience.
Don't forget, there's a lot more to the split besides what we see on TV. It's also an excellent decision from a business perspective.