All of the above is true, you may not need it. But if you are having a vibration problem I'd go back and insist they do it. Its one of the only "objective" ways to find a bent wheel or a tire with a manufacturing defect. If you have a bad tire, it needs to be discovered within a short period of time for the mfgr warranty to cover replacement.
In general, I have found with America's Tire/Discount Tire that they are allergic to their Hunter Road Force balancers. I have had a hard time getting them to use it, even when I was having obvious vibration issues, and even when I was willing to pay extra. Not sure if they aren't trained on how to use it, just don't want to take the time, are afraid of having to warranty more tires, etc... I'm guessing that on most family cars with soft suspensions, road force balance isn't necessary unless the tires are really bad. But on my BMW I have found it make a big difference.
My last set of tires from them I had to return 3 times due to bad vibrations before I could convince them to use the road force machine. They subsequently found 2 of the 4 tires they sold me were bad from the factory and I had a bent wheel. Had they just road forced them to begin with we would have figured all this out in one visit and I would have been a happy customer. Instead my time was wasted with 3 return visits.