As a designer, I'd say this is correct. Many of my weeks consist of coming up with concepts, prototyping them quickly, and testing designs on users. The faster I can cycle through this, the better my designs are for users.

In other words, designers favor tools that let them prototype a high volume of designs in a short period of time. Framer lets me work faster than jQuery, and at a higher quality than tools like Axure, Fireworks, OmniGraffle, etc.