Is there a benefit to leaving out a poor defendant from a suit and only suing the rich defendant?
The only benefit that comes to my mind is marginal: the plaintiff will have to serve a copy of his filings (think of pleadings, motion briefs, notices, subpoenas, and records stemming from those subpoenas) to just one party instead of multiple defendants.
The issue of being co-conspirator or agent of the rich defendant is crucial to whether suing the poor defendant would be futile.
A finding of conspiracy is precluded if the poor acted in his capacity of rich defendant's agent. See, for instance, the excerpt just prior to the Conclusion section in McGary v. Williamsport Regional Medical Center, (3rd Cir., June 6, 2019):
"[a] single entity cannot conspire with itself and, similarly, agents of a single entity cannot conspire among themselves." Grose v. Procter & Gamble Paper Prods., 866 A.2d 437, 441 (Pa. Super Ct. 2005) (alteration in original) (quoting Thompson Coal, 412 A.2d at 472); see also Rutherfoord v. Presbyterian-Univ. Hosp., 612 A.2d 500, 508 (Pa. Super. Ct. 1992) (explaining that employees and a corporation are "one and the same" when employees "act within the scope of their employment").
If the poor's wrongdoing exceeds or departs from what would be the reasonable expectations as to his employment, then he may be sued in his "personal capacity" (not sure whether that term is also used in complaints that are not against a government). Absent that excess or departure, he would be easily dismissed as defendant to the case.
could the jury end up assigning too much of the award to the poor defendant?
It depends on whether the wrongdoing involved conspiracy. If it did not, then the liability or contribution would be prorated in accordance to each defendant's role in the unlawful act(s).
But when the poor and the rich defendants' meeting of the minds and coordinated wrongdoing are cognizable as conspiracy, there is no risk of the poor defendant being ruled as liable for too much of the award. As noted in Lorillard Tobacco Co. v. Alexander, 123 So.3d 67, 80 (2013),
the law regarding conspiracy is well-settled, and provides that an act done in pursuit of a conspiracy by one conspirator is an act for which each other conspirator is jointly and severally liable.
(joint and several liability means that recovery is obtained from whoever [among the defendants] has resources to pay to the plaintiff)
Even if there is neither agency relationship nor conspiracy between the defendants, suing the poor should not be discarded merely on grounds of his economic status. Being poor hardly ever justifies engaging in unlawful conduct, and the plaintiff has the right (which at the same time I see as societal duty) to spotlight all wrongdoers regardless of each individual's finances. That tends to be more effective toward dissuade any wrongdoer --rich and poor-- about subsequent misconduct than if not taken to court at all.