It seems when I talk with people I'm surprised they use ultralight rods and light line for river smallies. And of course they are surprised I use what they consider much too heavy of a line size. For me, I see no reason to use anything lighter than 8lb test, on an exceptional rare occasion maybe 6 pound, but that's the absolute lightest. Ultra light rods and light line make it easier to break off and possibly kill the fish, or work the fish to exhaustion in the summer when the water temps above 80. I once saw a guy with a fly rod at Cardiac hill below Meramec springs who told me his favorite tatic in the summer was to use a very light flyrod and fight the fish for a long time, "It just makes my day because it's more fun to fight the fish for a long time". Didnt want to burst his bubble but should've said something.
My biggest smallmouth in Missouri came on 15 lb test in the middle of the winter, but I admit that was overkill becuase I was fishing a larger jig.
Actually I can't think of one reason on the river fishing for smallmouth where an ultralight rig is appropriate. Yes, if you're fishing a lake in the winter for suspended fish with a float and fly, light line would be ok, but a river is something completely different.