I always assumed he would be back at some point!

From the DiS interview:

DiS: One of the longest serving members of Primal Scream up until 2006 was guitarist Robert 'Throb' Young. Do you ever see yourself working with him again in the future?

Bobby Gillespie: No, it's never gonna happen again. That moment has passed. I don't want to get into the reasons but things turned out how they did. He went one way and we went another way. He stopped making music and we carried on making music. You just have to get on with your life. Robert Young's an incredible musician, an incredible talent. One of the most talented people I've ever known. I grew up with Robert and I love him like a brother, but we don't see him any more. I don't think anybody does. I don't really know what's happening there. I don't think he's making music now. But I wish he was because he's super talented. But then at the same time, we've just made a new record which we're really proud of so I don't see the point of looking to the past.