Think you've summarised perfectly.........Brian, for all his shortcomings, for all the obvious nonsense that he speaks, is only there on a regular basis because RS encourage him. Brian often appears early in the show, and it's because he's said something outrageous, or at least controversial, that RS know that it will provide enough momentum for the next 30 minutes or so. I actually find it amusing how RS invite Brian to say his piece then quickly show him the door, simply because all it takes is one piece of nonsense from Brian to stimulate further phone calls, and thus the show justifies itself etc, etc.
The trouble with all of this is that it encourages Brian to imagine he's got something of worth to say, so what actually happens is that Brian assumes an increasingly outrageous/stupid/incorrect role as phone-in fool. Brian you're not that interesting, you just happen to have discovered a platform that gives you the celebrity you've always hungered for. Hopefully, one day someone at RS will appreciate that Brian has little to say that is genuinely interesting, thus he'll be relegated to a place where he assumes his most appropriate position of village idiot.