Reasons already explained: the phrases that it introduces have an implicit agency that the true prepositional phrase can never have. Simply to label it a 'preposition' (which, admittedly, some reference works do) is, to my mind, sacrificing accuracy for the sake of simplicity!
There are so many dictionaries considering "AS" as one of functions of "AS".
preposition 1 used to refer to the function or character of someone or something: it came as a shock. 2 during the time of being.
preposition time when: used to indicate a stage in somebody’s life
As a teenager I was quite shy.
- http://encarta.msn.com/encnet/features/ ... 1861587033
Definition as (FOR THIS PURPOSE) [Show phonetics]
used to describe the purpose or quality of someone or something:
She works as a waitress.
It could be used as evidence against him.
The news came as quite a shock to us.
I meant it as a joke.
http://dictionary.cambridge.org/define. ... &dict=CALD
Definition 1. in the function or position of.
Example He is serving as temporary director
http://www.wordsmyth.net/live/home.php? ... 2CPHR#POS7
in the role, function, or status of: to act as leader.
appearing to be, or being
He went to the Halloween party dressed as a banana.
As a child, Miriam had lived in India.
The news came as no surprise
http://dictionary.cambridge.org/define. ... 2+0&dict=A
and so on....
What do you think?