Hello everyone,
I am trying to understand why carbon fiber components in BMW 7 series and many other cars are used in conjunction with Steel? Why are carbon fiber stand-alone components not used instead of these hybrids? Are there any intrinsic problems with stand-alone carbon fiber components? 

The biggest drawback that I can see is that once carbon is broken, it's broken. It has an elastic performance near 0, whilst steel might be heavier but it maintains very good performance past its tensile strength due to its high elastic performance. In an application like a vehicle pillar, this could be extremely important in ensuring crash safety. Say in the event of a roll over for example, carbon fibre pillars are very likely to break and give no more strength to the vehicle structure whereas steel will continue to absorb energy and is unlikely to completely collapse. 

The hybrid structures are likely based on the idea that the steel is there to absorb impact energy in a crash situation, whilst the carbon gives rigidity and lightweight performance in everyday situations. As for the cars utilising solid carbon structures, I'd hazard a guess that those manufacturers have completely redesigned the crash structure of the car so that it dissipates energy to other areas in a crash and doesn't rely on the pillars for protection.

Of course costs vs design spec are also going to be factors in this as well.