It's used! Examples: I had been wanting to go abroad for a long time, and then, in 2005, I got the chance. My son had been wanting desperately to buy a car, but could never find the money. Then, last year, he won the lottery. Mike had been wanting to learn French for ages, but couldn't find a teacher. Then last month he discovered italki, and now learns online. These could be phrased in the past perfect simple, 'had wanted' - but the use of the continuous helps give the impression of duration/repetition. We can do the same thing with the PRESENT tense. Present perfect: I have been wanting to tell you for a long time that I don't like you! Finally, I've said it! So there! Present continuous (mother talking about baby): Oh, he's wanting his dinner! That's why he's crying!

"Had been + verb+ -ing" makes up the past perfect continuous tense which is totally OK and in use. Regarding the -ing form of 'want,' it could occur in the present perfect progressive and the past perfect progressive, depending on context. Examples: 1) Oh, thank you for your gift! I've been wanting the new car for a long time. 2) Marry had been wanting a Barbie doll since her last birthday, but when she finally got one, she didn't like it.

They **had been trying** to conceive [to have a baby], but their efforts proved fruitless. This tense is called the past perfect continuous: http://www.englishpage.com/verbpage/pastperfectcontinuous.html To want is not generally used in the continuous tenses, though; you just use the regular [i.e. non-continuous] tenses with it.