I've done several patent trials as a lay/fact witness in cases involving patents related to an employment contract addendum I created when I worked for Amazon. The contract described various early web tech hacks (such as embedding session IDs in the URL) and denied Amazon the chance to patent them (0). In the first one, I was in court while the plaintiff's damages expert witness (there to estimate how much this large…
That he was paid is not enough reason to invalidate his testimony. Everyone in court that day, the lawyers, the judge, you, were paid. Why not him? If he indeed has expertise that would win his party's case, what exactly is wrong with getting paid, even paid handsomely, for his being there? This seems a lawyerly sort of ad hominem to discount expert testimony without directly challenging the evidence.
It's a person, nominally an expert, saying what they think/know and providing some expert-y looking handwaving to back it up.
You've got two choices (at least) as an attorney for the opposing party: one, get into a domain turf war with the "expert" about whether what they've said is correct; two, discount the "expert" personally.
The problem with the first approach is that (at least in the case I was describing), there was nothing actually wrong with what the expert witness had said. If it actually was the case that the defendant had infringed on some validly held patents, then the expert's damages estimate was probably pretty accurate. The dilemma is that the defendant's whole case is based on having not infringed and/or the patents being invalid.
Consequently, there's zero motivation to get into the details of what the expert said, and every motivation to do something to as much as possible erase it from the minds of the jury.