I have a IR231C 1/2" gun that's 600Ft lbs, and I'm getting a HF earthquake 3/8" gun that's 300 Ft lbs. My "impact driver" is rated for 60 ft lbs...so yes 3/8 impact wrench is worth it, and is NOT just a duplicate if an impact driver with an adapter as long as you're getting a decent 3/8"
I'm getting one because 1/2 is too much balls for some applications, won't fit in some applications, and is lighter for extended work. I do have a 1/4" hex to 3/8" square adapter I use on my impact driver for very light work, but it's not ment for the kind of abuse my 3/8 impact wrench can take.
1/4 hex impact driver = screws, lag bolts, small stuff around house and car < 25 ft lbs (half of max)
3/8 impact wrench = general automotive use, large lag bolts, small engine use, and mower blades
1/2 impact wrench = automotive use on large or rusted fasteners, suspension work, crank bolts, tires, farm maintenance, heavy equipment maintenance up to 3/4-1" bolts.