(Going to clarify a bit: If the purpose is to communicate better with your kids before they can talk, it's a waste of time. Learning ASL is a valuable thing to do for many other reasons.)
In our case the kid picked up signs we were teaching him (not ASL, just intentional signs) weeks before corresponding words, and there are still words he signs and understands but can’t pronounce. The point was to introduce expressive language for functional concepts (hunger, wanting help, wanting to be picked up, etc.), and beyond its role as a precursor to spoken words, it absolutely made our lives easier.
It’s a well known phenomenon that the ability to pronounce words lags behind the ability to express “language”. Your kids clearly didn’t need it, but it’s still a useful tool.
Using ASL signs as the base means you may have a better chance of others understanding, which is great (unless you're using signs to communicate base running strategy).
Lots of kids can communicate a little earlier with hands than speech, and there's lots of situations where hands are visible but speech is inaudible. More tools for communication is usually better.
My kiddo is 13 now, and speech is usually more convenient, but we still use a couple systematic signs (shaken t for toilet, all done), and the experience of looking for signs is helpful in a crowded place even when using ad-hoc signs.
That said. Like all parenting advice, try things out, if it sticks, keep using it. If not, no worries, nothing fits all.
Oh, I totally get that. But SL makes it that much easier.
> Don’t feel like you have to follow this advice.
I wasn't commanding anyone to do something. Good Grief.
That said, there's a marked difference between the kinds of assymetrical communication that you're describing and baby sign language. We've done sign with each of our three children and it's remarkable to watch them pick it up—they clearly understand that these hand gestures are words that carry specific meaning and get a lot of delight out of learning and applying them correctly and being mimicked and understood. The language center of the brain clearly develops much faster than the parts responsible for controlling the tongue, and sign gives them a great outlet for that language.
Also, you don't have to do anything complicated to give them just a little bit of sign, so unlike a lot of parenting advice it's not difficult to try out. Just say a word while making a sign (ASL if you want but we'll sometimes just make up one on the spot) and then give them the thing or do the thing. "More", "water", "milk", "diaper", "good night", "up"—just pick one and try it! Once you start and see their delight it's hard to stop.
As one of many, many parents, my observation has been that there is so much contradictory parenting advice because there is so much variation in the kids, in terms of behaviors, personalities, the order in which they bootstrap their motor vs verbal skills, body awareness, etc., etc. As a result, consuming parenting advice is like going down a checklist of things to try, and the kid won’t respond to most of it.