Castillo has multiple skills, which mitigates the risk of a flameout. Even as a mediocre hitter, he can still provide value with defense and baserunning. Tomas needs to hit to be a valuable player, and hitting is the toughest skill to assess without reps against MLB-caliber pitching.
The glut of speedy OF talent isn't much of a glut. Victorino is often-injured and under contract for one more year. Cespedes is under contract for one more year, and is more of a corner OF. JBJ may never hit enough to be a MLB regular, even with his otherworldly defense. Mookie and Holt are both infielders playing the OF to get their bats in the lineup, and either could be needed at 3b depending on how that position ends up. Nava is a limited player who's probably never going to block anyone.
Even a Cespedes/Betts/Castillo/Victorino/Nava (JBJ, Brentz in AAA) OF isn't exactly stacked. If Tomas is the answer in LF or at 1b/DH, that's an entirely separate concern. Given Napoli's contract status and Ortiz's age, it's not like those positions are locked down forever, either. There's room for Castillo and Tomas. It's not an either/or proposition.
