If nothing else, having him on board will make more people take notice of Rift. He is one of the biggest names in gaming. It's like have Stephen Spielberg as your film's Executive Producer - it's good even if he doesn't lift a finger!
From my point of view, this is very exciting. While RAGE was one of the most boring games I have played for a long time (purely because I had played Fallout 3 and Borderlands already, before playing RAGE) the engine was out of this world. Now, if you get that engine working nicely with Rift, that would be amazing. Id tech 5 also really nicely integrated with consoles... it was actually nice to see such good graphics on the consoles... will that integration work well with Rift? Are we taking Rift to the PS4 and XBone?
Even if you take none of his tech and previous work along and we don't touch id tech 5... John will bring a wealth of knowledge and hopefully make the Rift grow even bigger. The more people who buy in, the more support for the device there will be, the more games we will see moving forward.