Earlier quoted context omitted.
“The books, she is green” is not very unique to Arabic actually. It's also how you'd say it in Polish, for example. Most languages, that have feminine and masculine forms (unlike English), have some arbitrary rules as to what form to assign for different material objects.
You've misunderstood this part. The unusual behavior is not having grammatical gender, but treating all non-human plurals as feminine singular.
Feminine genitive plural happens to have the same suffix as feminine singular, and it could be possible to envision an future evolution of the language where the genitive is lost, while the number agreement rule could be preserve. So it's possible that grammars of this future language would state that feminine plural agreement for numbers >=5 you have to use the feminine singular.
Btw, I once read a very nice title of a german language book:
"Der Dativ is dem Genitiv sein Tot"
literaly, "the dative is [to] the genitive its death"
Which denotes the fact that in modern German the genitive case is being lost in faviour of constructs based on dative (the correct form would be "Der Dativ ist der Tod des Genitivs", i.e. "the dative is the death of the genitive")
(http://en.wikipedia.org/wiki/Der_Dativ_ist_dem_Genitiv_sein_...)