The style question
aimed in part at boosting Mansoor's legitimacy as leader in the face of opponents within the movement.
The Original Poster wonders if we could just use the leader here instead of leader. They rightly say that leader can be used on its own because it's a title. This is close, but a better way of putting it is that it indicates a special unique role.
Now one of the ways that we know what leader refers to here is actually that it has no article! Because there is no article, we know that leader represents a specific special role. The only official leadership role that has been discussed (if you read the rest of the article) is that of head of the Afghan Taliban, so we know that leader refers to this position.
Now, if we say the leader instead, this isn't necessarily a special role:
aimed in part at boosting Mansoor's legitimacy as the leader in the face of opponents within the movement.
It is not specific enough to evoke a special role - and the use of the article means it doesn't benefit from being clearly portrayed as a clearly specified role by the grammar either. It needs something to clearly delineate what Mansoor is supposedly the leader of, to make it sound elevated in that way. This sense of elevation is important here because what Mansoor is trying to do is establish his status. Now we could say the leader of the Afghan Taliban here instead:
aimed in part at boosting Mansoor's legitimacy as the leader of the Afghan Taliban in the face of opponents within the movement.
This would make the role seem more like a proper title and give it the sense of status that is required.
However, this is in fact not the OP's most interesting question at all! Cookie Monster is right that one of the reasons that we can use leader here without an article is that it describes a special role. But that is not enough! We cannot just go dropping articles right left and centre when we are talking about a special role. The following sentences are all completely ungrammatical:
- *I punched Managing Director.
- *Master of Ceremonies drank too much.
- *I hate Best Man.
- *President is an arse.
When we use these singular nouns without any articles in the sentences above, they are completely ungrammatical. So why is Cookie Monster's sentence grammatical?
So Cookie Monster's big killer question here is:
- Why can we use leader here without an article?
The Grammar Question
To understand what's happening in the Original Poster's example, we need to look at Predicative Complements.
What's a Predicative Complement?
A Predicative Complement is usually the complement of a verb. More specifically, Predicative Complement are phrase that describe the Subject or Object of a verb. Predicative Complements can be adjective phrases, preposition phrases or noun phrases.
Some examples of verbs that take Predicative Complements are BE, FEEL SEEM and ELECT:
- He was [a clown]. noun phrase
- Bob felt [happy]. adjective phrase
- The president seemed [in a hurry] preposition phrase
- The committee elected Mary [treasurer]. noun phrase
In the first sentence a clown describes he. Notice that there is only one person in this sentence. Predicative Complements never introduce a new person or thing into the conversation. Compare that sentence with He shot the clown. In this sentence the clown is an Object of the verb and does indeed represent a new person. In sentence (2), happy describes Bob. The phrase In a hurry describes the president in (3). Example (4) is different. Here treasurer describes the Object of the verb Mary. It doesn't describe the committee. Notice that there are three noun phrases in this sentence but only two entities, the committee and Mary.
Bare role noun phrases
Some verbs that take noun phrases as Predicative Complements, can take a special sort of noun phrase. These noun phrases describe some special role that someone has, for example President, or student of the year, or Best Man at my wedding. They are special because they do not have any articles. We don't need the words the or a when we use these nouns - even when they are singular. We don't need to use any determiners at all. These types of noun phrase are called bare role noun phrases (bare role NP's). They are bare because they have no articles. They are role NPs because they describe some kind of special role that someone has.
Now many students know that verbs take Predicative Complements. But not so many know that some prepositions also take Predicative Complements too!
Compare these two sentences:
- Bob worked for the Managing Director of IBM.
- Bob work as Managing Director of IBM.
In the first sentence the Complement of the preposition for is the noun phrase Managing Director of IBM. Notice that this preposition is introducing a new person into the conversation. The Managing Director in example (5) is the second person in the sentence. There are two people here. In example (6), however, there is only one person. The phrase Managing Director just describes the Subject of the sentence. When we use the preposition as like this, it can often also take a bare role NP as a complement. If you look at example (6), you will see that it says Managing Director not the Managing Director. This is only possible because Managing Director of IBM is a Predicative Complement of the preposition.
Different prepositions taking Predicative Complements
The most common preposition taking Predicative Complements is definitely the word as. Here are some more examples from The Cambridge Grammar of the English Language by Huddleston & Pullum (2002):
- I regard their behaviour as outrageous.
- As treasurer, I recommend we increase fees by 10%.
And some more of my own ...
- I think of her as a friend.
- The noun phrase functions as Subject in that sentence.
There are a few other prepositions apart from as that take Predicative Complements. Probably the most common are for, from and to. Here are some examples:
- He took me for an idiot.
- He went from Chief Bathroom Assistant to Managing Director in less than a year.
Notice that in the last example, I used bare role NP's as the Predicative Complements.
The Original Poster's question
The important part of the Original Poster's quote is this:
aimed in part at boosting Mansoor's legitimacy as leader
Here the word leader is the Predicative Complement of the Preposition as. Notice that there is only one person being talked about. The noun phrases Mansoor and leader refer to the same person. Because this noun phrase describes a specific role and is also a Predicative Complement, it takes the form of a bare role NP. Because of this we see no determiners, such as the word the here.