Is a poorly designed website better than no website for a commercial enterprise?

For businesses, a website is a channel of communication. For all except web-based businesses, it's not the only channel of communication available.

Given that anyone could make a DIY website for next to nothing, should they be choosing that option as a business person looking for the most cost effective solution? If not, why not?

How important / useful is a website for brand identity?

Is it better to save money to invest in a professionally designed website, rather than spending a third of that for a poor result?

I'm asking these closely related questions because I'm looking for ways to explain to potential clients that a 4 figure (£) website is worth the investment, and pre-emptively explain why it's not as cheap as GoDaddy makes it seem, if you want a professional website.

Most large companies fully understand the value of websites et al., but a fair amount of medium sized businesses, and a lot of small business tend to require a lot more explanation and convincing. They're not naïve at all, just that the internet has grown so much so quickly, and I personally think quality design services are drowned out by the vast amount of cheap, budget design services all over the internet.

When I was interning at a marketing agency, they had a smooth talking executive director whose job was mostly convincing / seducing clients to invest in multi-million pound projects. I never got a chance to learn anything from him though.

How would you explain the advantages of a professionally designed website over a cheap DIY website?

Is it okay to be confident with stating that a poorly designed website is likely to do more harm then good?