Not necessarily. The Daytona and Superbird were all about aerodynamics. In the wind tunnel they had discovered to achieve the desired stability a rear spoiler would need to be over the 3" tall NASCAR limit and would produce lots of aerodynamic drag. The wing gave the desired downforce without the drag of a too-large illegal spoiler. Plus as a side benefit the tall vertical sections acted as twin vertical stabilizers like on a plane and helped keep the rear of the car stable and in-line. I believe aerodynamics was the first concern, once designed it would be nice if one could access the trunk. These cars were built for one reason and one reason only, the race track. It is just that NASCAR required that a quantity of street versions must be made to qualify them for NASCAR.
http://www.allpar.com/model/superbird.html
Two stories explain the rear spoiler. Some wrote that it's three feet tall so the trunk can open. An interview with the engineers at Chrysler, long ago, included a comment to the effect that as the numbers kept getting better as they raised the spoiler, until it was three feet off the car, that's where they left it. Either or both could be true.
“The ‘69 race Daytona package got the front lift of a standard Charger to zero, from 1,200 pounds at race speeds. A bigger front spoiler, further forward, would make some downforce – relative to a no lift condition. The rear wing would be used for down force and to balance the car, aero wise. To get the same effect with a rear spoiler would cause a lot of drag! The spoiler would have had to be well beyond the NASCAR limitations of 3 inches.
In early 1965, I rented the University of Michigan wind tunnel for extensive tests. I hired University of Michigan-associated aerodynamicist Jim Amick to manage the tests and compile data into a report. Ron Martin built the wind tunnel models, and worked with Amick between runs making changes.
Amick's final report had a unique solution...The Wing.
Amick’s March 1965 report included the following: “The divergence speed for a given combination of spring rates can be increased indefinitely by the installation of a horizontal airfoil at the rear of the vehicle. A convenient place for such a stabilizer might be above the rear deck. An airfoil of 15-inch chord mounted 20 inches about the rear deck and spanning the full width of the car would probably provide complete aerodynamic stability.