Hi Dave,
I don't have the smoother, but I do have the low angle block plane, which also has an adjustable mouth. It isn't a gimmick at all, and makes adjusting the plane very easy. I'm no plane expert, but I've gotten bit by the bug and have only in the last few months purchased a Clifton #6 (on sale at Woodcraft, too good a deal to pass up) and a load of eBay Stanley's (a #3, 2 #4's, #5, #5 1/4 and a #7). I also have a Knight coffin smoother and Knight razee jack plane. I'm also getting the Veritas scraper plane for Christmas, DAMHIKT :).
Anyway, my point is, on the bedrock style planes, you have to set the frog properly to fully set the mouth width. Again, I might not be using the correct terminology here, but hopefully you'll see what I'm saying. When you adjust the iron to the depth you want, if the mouth opening isn't the way you want, then you have to readjust the frog - which isn't hard, but it's just one more thing to do. On the Veritas block plane I have, you just open the mouth all the way, set the blade depth how you want, then move the mouth to the opening you want. It's a great thing, really.
Mike