This is a major consideration on the international scene with purely proportional systems. It seems to obviate...and I think some solutions were found, but I can't remember what they are.
In the mixed-member system, it is not an issue, because you can still run as an independent in this particular case.
In the example I've given, you could also have independents. Their chances of being elected, by the way, would probably be the same as they are right now. We can't devise a system—at least, not readily—in which independents would rule the day, but it is important for that phenomenon to be able to express itself under our system, and there are various systems that would allow that quite readily.