This avoids potential double litigation and possibly inequitable outcomes.
At the same time, if the patent owner is not a party to the case, the alleged infringer could be sued separately by the patent owner, and could end up having to pay two judgments for the same act of infringement.
In diversity cases, which brought in federal court on the basis of all plaintiffs coming from different states as all defendants, joinder will not be deemed feasible if it destroys diversity.
In some jurisdictions, the failure to join an indispensable party does not hinder the case.
For example, the Commonwealth of Virginia does not recognize the doctrine of indispensable parties.