Well, having a strong core can help a pitcher develop more velocity. I don't do any ab workouts. I just focus on compound lifts like sumo deads and squats. Also, I do sprints, etc... Generally, with a solid program, and good nutrition habits, I see no reason to do an ab workout every night.
But, if you would like to vary your routine up, and maybe do something different, you could do a medicine ball workout specifically targeting certain pitching motions.
In the off season, I add front squats to my routine, and perform that on back day. Trust me, after doing these, there's no reason to waste time on crunches. But that's just my opinion, I'm sure others will be able to help you out on this.
Check out Mike Griffins site, and look at some of the exercises on there. noexcusesbaseball.com
He also has a program that you can buy.