My understanding is that the early, lighter cars (like my '69) didn't have that bad a problem (as built) with cowl shake. But by the 1980 or so, there had been so much weight added to the car in the form of big bumpers, interior bits, smog stuff, etc. that the original chassis was really becoming too flexible and needed additional stiffening. This was finally accomplished in 1982. By the time that the 4th series (91-94) cars were built, yet more weight had been added with the new interiors, A/C, power windows, etc. The factory probably should have added additional stiffening and changed the suspension somewhat, but they didn't.
So, the early cars can benefit greatly from the stiffener, especially because of improved tire technology which places more stress on the chassis, the middle year (82-89) cars benefit from the chassis stiffener for the same reason and because many folks have installed wider than stock tires and improved shocks, springs and bushings. The series 4 cars really NEED the chassis stiffener because of the greater weight.
In short...it's a good idea to install one.
Rbitzer, the reason you are "road-hopping" may have more to do with your shocks, springs and/or tires and tire pressures. What are you running in those areas?
Cheers,