As a relative newcomer I have been trying to figure this out. Why a smoothbore? What is the rationale?
Versatility plays a huge part, as you mentioned. But even more-so, heritage. The 18th century New Englander is descended from Englishmen. England (to include Scotland, Wales, etc) was not rifle country.
In Europe, rifles came from a fairly small area of Germany/Switzerland. When immigrants from those areas came here (mostly to PA) they brought the rifle with them and it simply grew from there.
Then there is also the fact that colonial militia laws REQUIRED a smoothbore firelock. At least one (NJ) actually fined you for showing up with a rifle.
So I get why the poor or frugal farmer probably owned some type of fowler, but then I open up Rifles of Colonial America and I see many beautiful, elaborate and even ornate smoothbores. At that level why were they so popular? Again, I can understand that a more well heeled individual might want a fancier fowling piece. but if, as an example, I look at the Hermann Rupp rifle in Volume 1. It is not a fowler it is a rifle, with a smooth bore? Why was that? As usual I guess I don't know what I don't know. Maybe I should get a smoothbore and find out what I don't know.
I can be argued that many "smoothrifles" were originally rifles, as stated.
Although they did exist in some areas/times:
"The Smooth Bore Rifle you mention would not answer for our trade...When the Indians use a rifle, it must be a real one, and they will not carry a Smooth bore of such weight so long as they can get a North West Gun."
President of the American Fur Co. to James Henry, 1840.
Mario