Special lines for holders of diplomatic/official passports are becoming scarce. That said, in many Third World countries, either type will get you some special consideration.

Contrary to what has been written here (and is commonly believed), a diplomatic passport does NOT confer immunity. The holder may be treated as if it does, but there is no legal protection. The holder must be acredited to the country involved to have immunity. Also, immunity may be waived by the country which issued the passport.

Contractors, I believe, sometimes can get an official passport with adequate justification. The standards are strict and I doubt if two weeks a year would suffice UNLESS you are going to a place which makes your life miserable if you don't have one.

An important point alrady made but worth repeating: it is often necessary to have a special visa in an official passport when a tourist passport does not require one. That means a lot of red tape for many countries.

In short, it's not worth it unless you are a very frequent traveler to certain countries.