We are a commune of inquiring, skeptical, politically centrist, capitalist, anglophile, traditionalist New England Yankee humans, humanoids, and animals with many interests beyond and above politics. Each of us has had a high-school education (or GED), but all had ADD so didn't pay attention very well, especially the dogs. Each one of us does "try my best to be just like I am," and none of us enjoys working for others, including for Maggie, from whom we receive neither a nickel nor a dime. Freedom from nags, cranks, government, do-gooders, control-freaks and idiots is all that we ask for.
We consider planks to be a basic in the calisthenic component of a fitness program. While many planks are really isometric, I'll put them under the calisthenic umbrella along with more active planks like Mountain Climbers and Burpees.
Regular elbow or straight-arm planks are good uses of "rest" periods between weight-lifting, too. Planks are not really cosmetic, for creating visible 6-packs. You will never do that. They are core-strengtheners, which means torso. A sturdy torso is one's core.
I probably do 8-10 30-60-second planks of various types weekly, usually as part of supersets. Not much, but it fills the time. I do plenty of other calis that make demands on core, but all exercise stresses the core because...it's the core.