Are Planks Really a good "Core" Exercise?

(Because they seem like a really good shoulder exercise)

Everyone is totally obsessed with planks, and some talk about it like it's the "ideal" core exercise, way better than crunches. That it's a perfect example of how to "work the core".

I'm not so sure though. . Here are some of my thoughts: 1. Lots of people tell me "my core is weak". When I ask them how they know this they tell me it's because they can't hold a plank for X amount of time. It seems as though the chest, shoulder and arms could just as easily be the limiting factor here. Maybe your arms are weak, and not your core. 2. If you have diastasis recti, your abdominal mucles might bulge down with the weight of your organs. Can this really be a good thing? Can it really help strengthen the core? Maybe it could make a DR worse.

Indeed, even if you don't have DR, if the muscles are kind of bulging down under the weight of your abdominal organs, are they really going to get flatter, or stronger?

3. Instead of doing a plank, why not just do a push up? The "core" muscles are doing the same static work (anti-extension of the spine), but you're also strengthening the triceps, chest and shoulders isotonically (not with static holds)?

4. If you can't hold a plank at all, it might be worth strengthening all the relevant muscles to build up to one (including the chest and shoulders). It may be worth starting on an incline.

But once we can do a plank, why not move onto more challenging exercises, instead of holding them for longer and longer periods of time?

Planks seem like a pretty useless exercise to me, when push comes to shove, and they definitely don't seem very useful for healing a DR. I don't include them in training programmes, but I do get trainees working on pushups, starting with an incline, while checking their abdomen for bulging.

It's not that they're BAD, (unless you have DR, then they're probs not great) but they just seem like a waste of time. There are far better exercises you could be doing instead.