Which improves your chances of getting a retrial. Judges do get almost complete domain over their courtrooms, but if they make an error in judgement that sways a jury to a "potentially unjust" verdict, that is grounds for retrying your case.
Allowing an expert witness to testify and refusing to allow a dissenting expert witness to testify is one of those situations.
While I agree with you in principle, it doesn't necessarily hold true. You need to ask another court to declare a mistrial or for other kind of appeal. They can also refuse. You are effectively asking one judge to tell another that they are ignorant of common law.
You also have to be able to afford this procedure, or figure out some way to represent yourself. You might think you can afford it. While inmates are legally afforded access to law libraries, it's basically a joke from my understanding. Something like no more than 1 hour a week.
Allowing an expert witness to testify and refusing to allow a dissenting expert witness to testify is one of those situations.