As a designer of HyperCard stacks, why should you be interested in developing your stacks in such a way that they work correctly with HyperDA? The simplest reason is market size. Millions of Macintosh owners with 512K systems are effectively shut out of the HyperCard world by the limits of their machines. Yet many such systems are in use in small offices, educational institutions and other organizations which are not likely to upgrade their systems soon just to take advantage of HyperCard.
But if your stack is one that such a user would fine helpful and would appreciate being able to access, designing the stack to be compatible with HyperDA will go a long way toward enabling you to expand the size of your market well beyond those people who own and use HyperCard on their larger-capacity Macintoshes.