I think people 'tend' to see one word keyword domains ranking for a number of reasons, and not because of the name itself - it's where people misunderstand causation and correlation.
1. People that own great one word keyword domains tend to be a bit more serious about them and invest more heavily in SEO and marketing.
2. One word keyword domains overall already tend to be older, and more established with more links than competing generics - age and link popularity are probably two of the most important ranking factors at the moment.
3. It's also worth noting that the anchor text of incoming links is a huge help. So keyword domains benefit from people linking to
Keyword.co.uk - Keyword Resources and Information., whereas brandable/generic domains don't benefit as much from people linking to
Generic.co.uk
I agree with people here that attributing a value to a domain based on search volume is a bit of a wayward way of going about things. Sure for some low volume phrases you're going to find it fairly easy to rank with an exact match domain. But for higher volume phrases in theory it's not really any easier than your competotors to rank - though in practice what you're paying for the age of the domain and any established backlinks and rankings that it already has.
As you may have guessed by now, I would consider myself more of an SEO than a domainer, though if I was starting a venture you would happily pay £xx,xxx for a great generic/brandbale domain.
Why?
Because you can never underestimate the value of a brand. Great looking short names are your brand online, the same has having a shop at a huge shopping centre, or a dingy stall down a back alley.
I'm far more likely to take mortgages.co.uk seriously that I am get-great-mortgages-online-now.co.uk, one looks like a spam site, the other looks like it's established and sticking around forever. - Of course great site design and fantastic unique content also all falls into this mix as well and is just as important.
On the subject of search engine results page click through rates, you're correct that the vast majority of traffic goes to the results at the top, but people do tend to recognise the difference between paid and natural results.
We have clients that are in the top three results for good one word search terms, both for PPC and natural results. We generally find that there is a 70/30 traffic split, with most visitors going to the natual results - though this does vary slightly depending on the industry.
ComScore data from Jan '08