Welcome, Y-o-u-s-e-f.
Firstly, the present perfect continuous is rarely used in its passive form.(Here's
one source. Many more state the same.)
Second, the reason passive perfect continuous sentences are rare is this. They are difficult to process. The best explanation for that, I think, is found in
Zellig S. Harris' operator grammar. The -
en of the passive (i.e., be
en) and the -
en of the perfect (i.e., writt
en) both have the same [semantic] source, something like
a state of The result:
Ex: A letter has been being written for two hours.
=> A letter
has the state of its being in the state of John writing it.
It's grammatical, yes. Just difficult to process. Imagine the thought processes of a person who's in the midst of forming a
passive present perfect sentence. Now imagine the listener or reader.
No reason such sentences are rare.
As for the reason English even has passive perfect continuous sentences, [/url=http://listserv.linguistlist.org/cgi-bin/wa?A2=ind9108a&L=linguist&P=1370]one linguist[/url] offers the following, working from the example
The car had been being cleaned (all week). He writes,
"[They] are restricted to contexts in which it is important to convey (in this case) that an event with a result was ongoing in the past but is completed now. Usually, we don't need to be quite that specific and we emphasize either that an event was ongoing or that it is completed."
Hope that helps a little.
Harris, Sellig S. 1982 .
A Grammar of English on Mathematical Principles. New York: Wiley.
Harris, Zellig S. 1990.
Language and Information. New York: Columbia University Press.