According to an inside source I spoke with earlier today, the real reason why WWE and Vince McMahon have decided to run the guest host concept on Raw is due to network pressure from Bonnie Hammer of USA.
The main feeling within NBC right now is that USA President Bonnie Hammer thinks Raw has lost it's "big star atmosphere." In looking at trending results, Hammer thought WWE's loss of Austin, Rock, etc followed by Shawn Michaels taking time off, and the brand split, has left Raw with only John Cena and Triple H as marquee names, with Randy Orton as the only new breakout star.
This is also why WWE is doing the "music video" version of Wrestlemania 25 on NBC, so that WWE can associate "more star power" with the brand. Bonnie Hammer was a huge proponent of the Donald Trump angle, but as predicted, WWE and Trump couldn't make the schedules work for long term for a longterm project, so Vince put the "celebrity guest" credit on Trump so whenever it's mentioned, it gets the rub from Trump. This is something that Bonnie Hammer is a big fan of.
The idea of Big Show vs Shaq on free TV instead of on pay per view is yet another example of WWE attempting to please USA and Bonnie Hammer, as normally Vince would book such a high profile match like that on PPV. However, with Bonnie Hammer being one of the few WWE supporters in NBC, and with the possibility that Hammer could very well one day become the next President of NBC, Vince McMahon, Stephanie McMahon and Kevin Dunn are doing everything possible to keep her happy.