Keyword research - how important is the number of ads?

I've just been doing some keyword research, which is not something I've ever been good at. I'm trying to learn as I go, and am looking for some keywords to build a site around.

I've found what look like good keywords, with a reasonable number of searches per month and not too much competition, but no adverts showing when I search in Google.

Now I have read over and over again that such keywords are a waste of time, as if no-one is bidding on them, they aren't any good. My question is, how hard and fast is this rule? Some of the keywords I've found look good, and seem to imply commercial intent (ie no "free" or such words).

I checked with Microsoft's online commercial intent tool, and it gave them all around 0.55, which is strong commercial intent according to the blurb on the page.

On the one hand, I'm quite tempted by such keywords as I find this part of IM very hard, and am happy to have found words with low competition, but on the other hand, there's no point in wasting time and effort on keywords that are doomed to failure before I start.

Thanks for any advice and comments you can give