Is mobility work a waste of time? That's a difficult question to answer. It really depends on your situation.
Years ago for good or for bad, I was a powerlifter and that's all I was. I had no hobbies other than training, eating, and resting. I was extremely tight and very immobile. Guess what, I didn't care.
Fast forward a bunch of years and that lack of mobility work came back to haunt me. I was very tight and had lots of imbalances. This culminated in a fair amount of pain. I was in desperate need of mobility work.
So little by little, I added in mobility work. I started clearing up issues and feeling better. I think part of the issue was that I kept adding in more and more work. If some work is good, a lot is better, right?
Here's an interesting thing I have noticed recently, I don't really need mobility work anymore. I live a pretty active life. I hike, paddleboard, ski, and do a wide array of other activities. All of these things offer a variety of movements on different planes.
If I'm able to do all of these activities and lift without any issues, then what do I really need mobility work for? Well, it's all over social media so I must need it. There are books and tons of articles out there. I constantly see new and innovative movements that I have to fit in my program. Nope, not anymore.
In our zeal to be better lifters, we have been sold on so many things that we need to be doing. The truth is if you are somewhat active you probably don't need much mobility work. So if you are a parent constantly playing with your kid, if you have a physical job, or are out just moving around for whatever reason, you're probably doing ok.
Those who need mobility work pretty much know it if they stop and think about it for a minute. If you can't get out of bed without shedding a few tears and or a fair amount of sweat, you probably need mobility work. If you are a die-hard powerlifter that does as little as possible other than lifting, yep, you will need work on mobility.
If you are unsure whether you need to do mobility work, I learned from Wendler there's an easy way to cover it. Throw in bodyweight work either as assistance or in a circuit for some conditioning. Do some rear elevated split squats, pull-ups or inverted rows, and some pushups. Just getting up and down off the ground is mobility work for real. Think about it. Getting up and down off of the ground requires balance, flexibility, strength, and coordination.
The goal of mobility work is to live a pain-free life and to be able to do the things in life that you need and want to do. Unless you are trying to be a yogi or contortionist, you may want to look into the amount of mobility work you do and the extent of which it's necessary.