As the ACR is the more track oriented Viper, SRT probably decided that the coupe body style was better suited given aerodynamics, chassis stiffness and safety (some clubs won't allow a convertible at all or unless you have a dedicated rollcage).
I agree that a convertible ACR would be fun. To me running a convertible at a track day has advantages over a closed car: I get a better sense of my speed, can sense when cars are near me better (fewer blind spot issues), better ventilation on hot days esp when wearing a suit and it's just more fun.
After adding safety equioment like a rollbar and 6-poits, I tried to get some of the benefits of an ACR by adding the Stoptech brakes, Sidewinder rims, etc. The thing I'm missing of course is the downforce which is a huge factor in the performance of the ACR. Still, so far, the vert has been really satisfying on the track. Still, this coming season I may add another set of ACR rims with the Sport Cups for dry weather days and keep the other rims with PS2s for cold or wet weather track days.
/Rich