Where in philosophy is using passive voice common? At least in Anglo-American analytic philosophy, first-person active voice has been the norm for decades, since the days when in other fields it was seen as too informal to start a sentence with "I". Even today, scientists love to write things like "three types of experiments were performed to test this hypothesis". While a philosophy paper is more likely to contain active-voice, first-person sentences like "I present an internal problem for David Lewis's genuine modal realism" [1].

As a computer science major with a philosophy minor, I personally found my philosophy minor much more useful in learning how to write clearly than my CS major was.

[1] Arbitrarily chosen example from a recent abstract http://mind.oxfordjournals.org/content/125/499/627.abstract