A portfolio is a standard way for potential employers to filter out designers who are capable of providing the level of results required for their company. However, if you're a Master or PhD holder then it's more technical based on your thesis done and written.

For any freshie, a portfolio can help you win a job and jobs for freelance work.

I don't believe in Flash portfolios because most people don't offer much value from it and it takes a while to load. If I'm sourcing for employees or freelancers, I'd rather glance at your work as quickly as I can and later call you in for an interview.

However, if you're aiming to be a Flash designer then by all means go crazy with Flash and strut your stuff.

cheers.