Flash Catalyst has facilitated a workflow of iterative design and development. However, Flash Builder remains a powerful tool and can be used to assist in prototyping. Prototyping has the advantage of giving us insight about end-users needs.
This is the topic of an article of mine published at Flash & Flex Developer’s Magazine, January 2011. Download the magazine for free here: http://ffdmag.com/magazine/1582-html5-what-will-html5-mean-for-streaming