Not useless against certain targets, such as ports during offloading in the middle of an invasion.
Name a ship that was damaged by a V2 or a unit that was delayed on its way to the front by one.
The Nazis would have had to launch hundreds of them to have the effect of a single Allied bomber raid, and they would have landed randomly across a target area miles in diameter. Also, the Allies were never presented with a similar target. A V2 type weapon would have been useless to them. It was almost useless for the Nazis.
JATO units are nice, but they were scarcely a vital part of our national security plan. Mostly the USAF and USN were interested in getting fully loaded bombers off as quickly possible before the bombs (from ICBMs) started landing. The strain on airframes was so great that they seldom used today...