The Plank Is Overrated
I'm a bit mystified about the apparent affection so many people have for the plank. Given the goals that almost everyone has (more muscle and greater strength), the garden-variety plank is a terrible tool to bring to the job.
I suspect the reason that many trainers have their clients do planks is for the same reason martial arts instructors have their students do jumping jacks – it's an easy way to burn time.
First, the plank is a static, not dynamic drill. That means its potential to stimulate muscular hypertrophy is pretty much zilch. As for strength development, the plank also earns low marks, since it's essentially an endurance drill for the core musculature.
It's Okay for Extreme Newbies
Granted, if you can't easily hold a static plank for 60 seconds, maybe it's worth adding to your program (or better yet, consider seeing a physical therapist). But if holding a plank for a minute is easy, it's time to move on to better drills, of which there are many.
A Better Choice
One of those better drills is ironically a dynamic version of the plank called "stir the pot."
Assume a standard plank position, but with your forearms positioned on top of an inflatable gym ball. From there, you draw imaginary circles with your forearms while simultaneously maintaining your balance on the ball. This is an effective way to challenge your core muscles and turn a plank-like position into a useful exercise.