Thanks to all who replied! As with many issues in real life and RVing, there are many different valid opinions. Certainly Dual Pane offers some advantages, even if slight. What really concerned me would a Montana without this feature "fit-in" as the norm and acceptable and the answer seems to be yes.
Out of about a dozen used Montana's I've found online for sale, only one mentions this feature.
One can always add professional or homemade "storm" windows if this becomes a priority. Perhaps a suspension upgrade would take precedence for example, or some other improvement like the second AC several have suggested. And fortunately, the ones I'm focusing on have slide toppers, so no need to enter that debate!
Have a great Memorial Day Weekend!