It's almost impossible to take typical motivational posters non-ironically after seeing the parodies, but I looked at that category anyway.
I take his point. Sure there are times that it is better to be silent than to speak. By talking too much you might cause others to think/realize that you don't know what you are talking about. You might interrupt someone who would otherwise have given you wisdom. You might embarrass someone by telling them something they could otherwise have been happily oblivious to. You could let slip confidential personal, competitive or military information. You might be talking about your cabin when you should be focusing on maintaining airspeed in icing while intercepting the localizer.
But if Publilius Syrus (Wikipedia and Internet consensus favour that spelling) regrets nothing he has neglected to say, then does he never think of what he could have done? I'm thinking of the first officer who didn't say, "Go around!" in Little Rock, or at least didn't say it in a voice loud enough to be picked up by the cockpit voice recorder, and of all the pilots who ever died because they saw something that wasn't quite right but assumed that the flying pilot had it under control, or that it would be safer for them and the cockpit atmosphere to remain silent than to nitpick a small discrepancy. I guess Publilus Syrus never knew anyone who committed suicide because he thought no one cared about him. Maybe Syrus was so irritating that he didn't have any friends, so none of them ever died leaving him wishing he had told them something. And if he followed his own wisdom, he never spoke out against injustice against others, even though he himself was a former slave.
Ironically it was Syrus' smart mouth that got him out of slavery. Had he remained a silent slave, his master wouldn't have been so impressed by his wisdom and entertainment value as to free him. It appears that Syrus became a professional maxim writer. I guess that's the equivalent of someone who makes a living from a cafepress store today. He was pretty good at his job, considering the number of sayings attributed to him that are still in use today, such as "a rolling stone gathers no moss."
I have no quarrel with "It is a bad plan that admits of no modification," (although if that plan is a published instrument approach, ad hoc modification is a very bad plan indeed). "If you wish to reach the highest, begin at the lowest," is true, but I prefer the way Lau-Tzu put it. Syrus strikes both sides of the coin with the complementary, "Nothing can be done at once hastily and prudently," and "While we stop to think, we often miss our opportunity." That way whatever you choose to do, you have a quotation to back you up. One of his sayings, "It is a good thing to learn caution from the misfortunes of others," is part of the slogan of the Aviation Safety Newsletter, from which I keep highlighting things I mean to blog about and then not doing so.
According to Wikipedia, he also worked as a mime. I must admit that as a reflection on a performance as a mime, those words are spot on. But I doubt he was so bad a mime that he kept forgetting not to talk. Another of his sayings, "Let a fool hold his tongue and he will pass for a sage," I think is a clearer expression of the same sentiment, but I still say speak out. If you are a sage, then someone can learn something from you. And if you aren't, someone else will likely speak out and help ameliorate your foolishness. And I suppose there are a dozen readers out there who silently think I'm a fool and should regard Syrus' advice.
And in other news, apparently some Indian pilots don't know how to fly. Indian reporters are having trouble reporting, too. Raw data flying is several steps removed from just turning off the autopilot, and if the pilots can't fly raw data in cruise, the chief pilot should be doing a lot more than sending circulars.