Selecting the right keywords to optimize a site is a difficult job and it becomes even more difficult when it is about long tail keywords. Is it important to focus on the length of the long tail keywords?
I would not say it's easy to rank well on long tail keywords. It just possible as all of the short catchy keywords have been "used". But I would agree, the length does not matter just the relevance on "searched per month" as shown by the Google Keyword Tool. I have always used 3000 searches per month or higher.
Since it is very much difficult to rank for the Most competitive keywords in beginning of SEO work on a website. So to get some quick visitors, although less but targeted, you should also plan for long tail keywords. Also it do not require a lot of work to do in order of achieve a good ranking for long tail keywords. You can not imagine what a user is typing to search for some specific thing, for example if someone is looking for a birthday cake in chicago he may type "where to buy a birthday cake in chicago", if you have targeted for "buy birthday cake, chicago birthday cake supplier" you may rank somewhere on top in the SERP.
LONG TAIL KEYWORDS ARE ALSO ONE OF THE SOURCE OF WEBSITE TRAFFIC. Think about it seriously if you have just started SEO of your website.
I think if you go for long tail keywords then it's little easy to get them on good position, the reason is they are no so competitive and the length doesn't matter but the relevancy comes at a prior level.
I agree that length does not matter. Long Tail Keywords are little easier to get SERP ranking. Also, the conversion rate would be higher than the short keywords, though there are limited traffic. Conversion is the main difference between short and long.