Well the more specific the search term (longer tail) the less likely it is that there is any data on that term. You cant be confident that a specific long tail keywords will produce anything if there is no search data on it.
IMO I find the idea of "targeting" specific long tail keywords kind of funny. By targeting head match keywords and generating content around those keywords you will naturally rank for long tail (high converting) keywords. But to think that I can anticipate the actual long tail terms seems unrealistic to me, unless it is something like a product specific "branded" search.
Does that make sense? Am I way off base here?