This is how a completely Adobe Flash based website appears to the Google’s search engine spider (your second most important audience segment). The snapshot below is from a real website and I’ve hidden the full URL to protect the innocent.
This means the search engine spider has no way of reading the content to determine what the site is about. The guys at Google are smart, but they can’t read your mind. Additionally, the spider has no way of navigating through the website to index the pages properly based on content. So, all the amazing content about your products, your service, your history, your people, your customers etc. is meaningless to the search spider.
In other words a site that looks amazingly awesome in Flash is rendered as a big old black hole to the search engine. That might be okay for some people but for most it is not and search remains king.