You want demand greater than 300 or 500 per keyword, not anything less.
While I agree, mostly, with what you're saying I'd like to add a bit, if I may. Depending on the niche, and depending on what you want to accomplish with your site, some of those 20 -30 demand keywords might be exactly
what you're looking for.
If you plan to sell some affiliate products, or your own product, those highly targeted keywords can be worth gold. Left side mirror for 67 mustang, is pretty specific, but remember you're doing a site on classic car parts
. Someone redoing a car may very well be looking for that exact part - and if your site happens to have an aff link for that part - instant sale
. That's only one page on your site, now put up about 1000 of them
. While that may seem like too much work and a waste of time - couldn't you just say 67 mustang parts put up your aff link and go home? You could, but likely you're not going to get as many sales. Likely this person has already searched high and low for that part, they're close to desperate
, give them that part, fast and easy and they'll be your new best friend.
While I still find it difficult to write out a site this way (sigh) it makes perfect sense to do it. You want to grab those folks that are in the "I want it right now" camp. If you make it easy for folks to find exactly what they want, you likely will get your sale.
I do understand that traffic is important, and you should check to make sure that altogether
your site will bring in enough traffic to accomplish what you want from it. For a site dependent on adsense, the more traffic the better, for an aff site, or your own product site - targeted customers are what you're looking for.