Hi Randy and Ross,
This seems to be a relative issue, and depends a bit on context (sort of like you seemed to refer to Ross). I live in and primarily hike in VA. There isn't usually much in the way of extremes here, whether wind, cold, rain, etc (the exception may be heat as it can be pretty darn muggy and hot at times). If i lived or hiked in CO for example, i might more fully agree with Randy.
Re: ponchos in general, if it for some reason did become too uncomfortably windy, or what not, i would probably tie some cord around the poncho at places to keep it less flappy. I always bring a bit extra dyneema type cord, since it is so lightweight and handy.
But i'm not against bringing a separate wind-shirt.. i don't consider or call myself a "ultra lighter", and since i'm on a definite budget i can't afford a lot of the gear that is popular on here. I did find a Nike wind-shirt without sleeves or a hood for 7 dollars new which only weighed like 2.4 oz, but after putting sleeves and a hood on it (Pertex Equilibrium fabric i got for 6.99 a yard), it weighs 5.4 oz., which is still not much in my book and i wouldn't be adverse to bringing it and a poncho, but i just don't think i would really need both. Next week i'm doing a 4 day solo hike in the Shenandoah area and hopefully i can go from the the southern most part to the northern most part (about a hundred miles).
For other folks who hike in more extreme environments (even like Mt. Washington and the White Mountains, which i occasionally go to), it might be a good idea to have both dedicated and separate windshirt and rain gear.