I think it is a wonderful tool. I don't necessarily think it is something every baby should be taught so that they will have it as a 2nd language when they are grown... but I think it is an extremely helpful tool for an older baby (6-9 months) who has wants/needs to communicate and can do so by signing instead of just shrieking and crying in frustration. That is what I used it for with my kids. My son was incredibly frustrated and loud at mealtime when I couldn't get him food fast enough (if he ate all of the original serving and was still hungry). Teaching him to sign what he wanted let him communicate with me without all the screaming. Made meal time much more peaceful for all of us.
Do I think I am smarter than parents who didn't use it with their kids? No. Not necessarily. I think a smart parent or "smart parenting" can be using the tools available to you to accomplish what works for your family. Signing worked for us, so we availed ourselves of it. Had I not known about it, I wouldn't have been able to use it, but that wouldn't have made me a not smart parent. And continuing to try to teach it to a child not "getting" it after much effort with no results, and frustrating them and myself in that effort, would be counter-productive.
I also think that it is patently unfair to a baby to teach them to communicate effectively with signing and then leave them with caregivers who do not know signing. That was my biggest stress at that time. If we wanted to go to a movie, I had to go over and over with the person keeping our son what "this" and "this" and "this" mean.... so if he does "this" then he wants _______. So if you are routinely leaving your child with someone else, then you might need to insure that the caregivers are on board with learning it too.
ETA: for the record, my kids were not speech delayed in any way. Our second born was talking before we even knew it (because we didn't recognize the sounds she was using for her brother's name right away--it evolved over time and eventually we figured it out, but she had been using a specific "word" for his name for a while already by then. Son talked pretty well and fairly early also. We dropped it after they could use words to communicate with us.