Quirk calls it a marginal preposition in CGEL, 9.8., p669.:
Some info about preps:...there are some words which behave in many ways like a preposition, although they also have affinities with other word classes such as verbs or adjectives
In the most general terms, a preposition expresses a relation between two entities, one being that represented by the prepositional complement, the other by another part of the sentence. The prepositional complement is characteristically a noun phrase, a nominal wh-clause, or a nominal -ing clause.Syntactic functions of prepositional phrases:
(1) POSTMODIFIER in a noun phrase:
The people on the bus were singing.
The people were singing on the bus.
In the afternoon, we went to Boston.
From a personal viewpoint, I find this a good solution to the problem.
In all fairness, she did try to phone the police.
(a) of a verb:
We were looking at his awful paintings.
(b) of an adjective:
I am sorry for his parents.Like adverbs, prepositional phrases may occasionally take a nominal function, for example as subject of a clause:
A: When are we going to have a meeting?
B: On Tuesday will be fine.
Such nominal use can be viewed as related to sentences that have been restructured so as to leave only the prepositional phrase:
B: (The proposal that we meet) on Tuesday...
The preposition can be omitted under the same conditions:
Tuesday will be fine. = Meeting on Tuesday... or The weather on Tuesday will be fine.
In addition to the functions of prepositional phrases mentioned in this chapter, we have a quasi-adjectival function as complement:
This machine is (very) out of date.
This dress seems out of fashion.
The adjectival nature of these prepositional phrases is evident from:
(i) their semantic similarity to adjectives, eg:
out of date = obsolete
(ii) their possibility of being coordinated with, or appositional to, adjectives, eg: They are happy and in good health. an old and out-of-order machine
their use as complementation also for copular verbs other than be, eg:
They seem in good health.
(c) Exceptionally (mainly in fixed phrases), an adverb or adjective may function as prepositional complement:
until now, in brief, by far, at last, at worst, etc.
(d) Prepositional phrases can themselves act as prepositional complements:
He picked up the gun from behind the counter.
The weather has been fine except in the north.
(e) some prepositions form a correlative construction with a conjuncion or another preposition, eg:
from six to seven
between NY and TXIt looks like we have a prep in the form of 'worth, but then:A definition of preposition
There are several points of similarity between prepositions and other word classes and constructions in English grammar, in particular conjunctions and adverbs, but also participles and adjectives.
Before discussing the marginal cases, it will be useful to try to define central prepositions.
CENTRAL prepositions in English can be defined negatively with three criteria.
They cannot have as complement
(i) a that-clause
(ii) an infinitive clause
(iii) a subject case form of a personal pronoun:
Training the dogs is worth that we make an effort.
Training the dogs is worth to make an effort.
Training the dogs is worth the money and energy you spend on it.
Training the dogs is worth they.
Training the dogs is worth them.
-- adjectives can be modified by adverbs, preps can't. Note the coordinated elements, which are: 'difficult' plus 'worth', an adjective and an unknown quantity.It is difficult (adj) but well worth the effort.
That is why the "marginal", I guess.