Although a defense expert witness testifies for a defendant, the expert can be cross questioned by the plaintiff. And an expert witness is not there to win the case for a defendant. Although the defendant hires the witness, the expert just gives accurate information about a topic that he or she is an expert in.
Sometimes, the testimony of a defense expert witness ends up benefiting the plaintiff as a result of cross questioning. So it's important to keep in mind that an expert witness is unbiased. Being hired by one party does not make an expert witness biased towards that party.