That’s just wrong information. You can go into VS mode AND pull for selected speed if you want. Thrust will drive the speed but your rate of descent will remain the same. As @PZL104 stated, OPEN descent can give you a much steeper descent rate than needed - not comfortable for passengers.
there isn’t a hard and fast rule. The FCU modes are TOOLS that pilots of the A320 understand how to use to achieve the desired result, depending on the situation.