Shaddows - 9:15 am on Feb 9, 2010 (gmt 0)
Using semantic analysis, you will get a boost for related keywords.
However, when you look at the sum total traffic from long-tail, and the conversion rate, you will likely decide the "main" keyword is not really important.
I have some "vanity" keywords ranking top-3, but they convert terribly. I have loads of mid- and long-tail phrases each bringing a fraction of the traffic, adding up to less than the main one, but bringing in orders of magnitude more money.
We're an ecom, and want the "Buy [att-1] [att-2] widget" types, but also have "widget". "Widget" has thousands of times the traffic, but results in very few sales. Sometimes, we pick up the main keyword "by accident", just by concentrating on the surrounding money terms