Dictionary definition:
One who has forsaken the faith, principles, or party, to which he before adhered; esp., one who has forsaken his religion for another; n : a disloyal person who forsakes his cause or religion or political party or friend etc. [syn: deserter, renegade, recreant] ; faithless to moral allegiance; renegade; a pervert.
Now if you just want to look at the first part, then some would be okay with the use of the word apostacy. Or apostate. But when you see the other derogatory words associated with this word..... faithless to moral allegiance, a pervert
the other thing that is added in
the WTS addition: "Others claim to believe the Bible but reject Jehovahs organization" . nothing in the NT applied apostacy amongst Christians to anything other than rejecting the teachings of Jesus Christ!
apostasy seems to have two understandings in the modern world 1) rejecting your religious denomination 2) rejection of the teachings of Christ(for Christians at least)