The obvious reason you might want him to have a license to speak in court is if he's blatently lying in court about engineering you can revoke the license after a legal proceeding and no longer have a paid liar as a tool for attorneys to hire to decieve the judge and jury.
Sure, in theory lying can have legal consequences, but only if you can prove the person is intentionally lying. To take away the license you'd only have to prove they are stating things no qualified engineer would say.
Suppose, for example, an engineer testified that bricks should never be used in housing because in his professional opinion bricks cause cancer. His source is a blog post as reliable as a supermarket tabloid.
How do you prove he's intentionally lying, rather than just insane or confused or stupid?
By making it a license issue instead, you at least move the goal as to whether his statements meet some minimum standard of professional expectations or care.
This is all just hypothetical, in practice I don't know to what extent these license authorities are supervising engineers. But ideally I'd like an engineer to at least have some pressure to remain honest when they testify in court.
