If you use Flash exclusively for your website, you will find some people just can't be bothered to wait for all the fancy stuff to load, so they quit within seconds.
Imagine somebody who has a dial-up connection and an old PC (or MAC) and design accordingly, to maximise your audience.
I wrote all my sites with Notepad, so I had full control of the screen.
One site (originally by a professional writer) had to be reverse-engineered and simplified because although it was "flash" it just did not work properly.
When I look at a web page's source code, I like to be able to read it, and not find huge chunks of system-created code.