To say BMW fans who are fans of Rolls-Royce's are only so, because the brand is BMW is a significant generalisation. It may be true in some cases. I think it's unfair though, to overlook the massive strides the Rolls-Royce brand has made under BMW, in a similar way to those made by Bentley under VW.
They are generally genuinely desirable cars now in their own right and that's in no small part due to falling under the control of companies that actually know what they are doing when it comes to car building, rather than the mess that was Vickers, or ending up in the convoluted mess that is BAE Systems.
My only interest in Rolls-Royce & Bentley Motor cars prior to the end of the 90's came from my father working for them. The core products weren't that desirable at the time IMHO, and were simply falling behind much less impressive benchmarks in a number of aspects. But, both companies started knocking out more desirable cars in the 2000's once they were no longer a relic of failing British nationalisation programmes. Had VW retained control of key elements of the Rolls-Royce brand, they may well have failed to achieve what BMW has with the brand, had they simply carried on with badge engineering jobs like the Seraph and Arnage, but we'll never know.
Maybach as an attempt at a standalone brand failed for whatever reason, Maybach as model range or trim spec within the Mercedes brand has been more successful and I don't think it's unfair to say that someone who likes an S-Class is quite likely to like a Maybach S, irrespective of brand name because it's simply an S that's more S than a standard S (if you know what I mean).
In any case, lot's of brands products improved significantly after being bought by other car makers, to acknowledge that doesn't make you a fanboy of the parent brand.