Old thread but...
If you look at the gear photo on Ummagumma you'll see that the controls area of the pickguard is heavily damaged. I imagine Roger got a Fender bass to replace the seriously-in-need-of-repair Ric and just kept using it, never bothering getting the Ric repaired.
Speaking as someone who has several Ric and Fender basses; I LOVE the tone of a Ric bass but there is something to be said for the overall simplicity of a P-Bass, especially live. I could see how someone like Roger, who views his bass simply as a tool, would gravitate towards a Fender bass over a Ric.
Here's a shot of some of my lefty basses:
