Here's my take on crawling and some of the questions or issues that have been brought up in this thread, which largely echoes
@Ricky01:
First, it's relevant to the OP because he wants a minimal way to maintain a baseline of "everyday" strength, and presumably be able to move freely and feel good. Crawling can definitely be an effective tool to accomplish this, and is a nice complement to the swings and cardio training the OP wants to focus on.
Second, the reasons to crawl are that in my experience regular crawling leads to generally feeling good and a "tied together" quality that carries over to lots of other activities. In my case it was a key to rehabbing from two rotator cuff surgeries and promoting better shoulder mechanics, such that my overhead lifting is completely pain-free and my lockout positions are better and more stable (I now have the mobility to support bells overhead supported by my structure and alignment without a lot of muscle power, which is something I did not have for many years). Even though crawling doesn't use full shoulder flexion like the overhead lockout, it was a key to unlocking that range of motion for me.
And that's the crux of the value of crawling to me. It's not a mechanistic, "stretch this," "strengthen that," "activate this," "use this energy system," kind of activity. Sure, you fire certain muscles and use energy systems, and it has some direct effects on those things. But I look at it more as nervous system input that has many and wide-ranging effects beyond that.
And BTW, I pay little attention to the "developmental sequence" theory behind OS. I think that Tim Anderson often oversells and overemphasizes the theory behind it and the idea of emulating and recapitulating the development of movement skills in children, and the idea that we were "meant" to...whatever. I don't necessarily buy it, and largely ignore it. To me, it's completely beside the point of why I use OS. I generally like to have a scientific understanding of what I do, or at least a theory/hypothesis that I have confidence can withstand a little critical thinking. With OS, I just don't care. When Tim starts talking about the vestibular system, or babies and how we were "meant" to move, my eyes glaze over. But when he comes up with another way to rock, crawl or roll around, I pay attention.
I just look at it in terms of black box cause and effect. It feels good when I do it, and when I do it regularly, I feel and move better. Specifically, my posture is better, my shoulder mechanics are better, and I feel both looser and more tied together overall. And I think a key phrase there is "do it regularly." I don't find OS practice to create dramatic immediate effects. But with enough consistently accumulated reps, "all of a sudden" everything feels better, and whenever I stop doing it for a while "all of a sudden" I don't feel or move as well.
I probably would have remained in more of a skeptical/critical mindset toward OS if I hadn't taken a two day workshop with Tim and Geoff Neupert. It wasn't anything they said or taught about it that changed my view. It was doing lots of it all day for two days. I did it and I felt good. That was enough empirical evidence got me to keep doing it, and I've continue to feel good when I do it regularly for several years now.
Third, I would discount the idea of a rigidly proscribed "proper" form. Tim promotes a playful, just do it, attitude toward OS in general and that is one of the things I like about him and about OS. To me, the big defining principals of OS crawling are 1) Butt down, head and eyes up. 2) Contralateral movement. If you have those two things, IMO you're doing it "right."