This is a question I've thought a lot about, so I thought that I would chime in.
It is rather ridiculous for a person who does not believe in the Bible to use it as an authority. However, the fact is that as a Witness, one is taught that only the Bible holds authority, and thus, it can be the only reference point when conducting an argument for or against a Witness' doctrine. This means that all other books / pamphlets etc. have nothing to do with the issue. The Wathctower is a man-made document that is not inspired. So also the Protestant and Catholic commentaries . . . so, the Bible must be laid from the Bible in order to convince or agree with a Witness doctrine.
The argument appears to me to be what exactly is a prophet. A prophet is one who serves to proclaim Jehovah's message. Your argument is that the relationship is different now than it was then. Thus, the people who were raised up by Jehovah during the time of the Israelites were inspired, but they are no longer. This is certainly true. It would be absurd to argue that the Watchtower is inspired - there have been many, many doctrinal twists and turns thus necessitating that it not be inspired. Thus, the writers are also not inspired and are not prophets in the Old Testament sense.
Now, it's important to remember that neither are any of the Protestant or Catholic doctrines infallible - they are also in a constant state of flux.
So, the question is whether the Watchtower is a false prophet or simply not a prophet at all.
Unfortunately, this is where the argument fails. If the Watchtower (as an organization) is not a prophet, then from where does it claim its authority? From its works? Paul refutes that this cannot be a basis for its authority. From its faith? Faith cannot be proven and is a question between Jehovah and the believer. Either one believes in apostolic succession or not - either there is a channel or there is not. If there is a channel, it must be the Catholic Church. Why? They are the only group that can trace its roots from early Christianity. But if one doesn't believe in apostolic succession, then one must assume that there is no fixed organization, but rather the originally Protestant idea of Bible authority and not man. It's hard to believe to me that Jehovah did not set up a constant 'governing body' if it were so important . . . Either he did or he didn't . . .
Thus, either the Catholic Church is right or there is no authority to claim an apostolic leadership.
On the other hand, if the Watchtower is a false prophet, then it is obvious that they are in fact a member of the great harlot and are worthy of death according to the Mosaic law. Who knows . . . I don't.
In the end, I look at it this way. I am not a Witness nor do I believe in the concept of Christianity as a religion. The Watchtower has made mistakes in the past. So have the Catholics and Protestants. Both are still making mistakes. But both help to make society better, perhaps in some cases but not in others. Wars are bad. So also are overhanded practices that destroy families and people's lives.
No person or group of persons have the right to take away one's self worth. This is what fundamentalist religion tries to do. As a Witness, how many times have you been told that you are unimportant? That you can't understand the Bible alone? That you need to be a part of an organization to be saved? As a Catholic, are you not told the same thing? Protestantism started well, but lapsed into sects and divisions and thus is guilty of the same.
If one is a Christian, one doesn't need to lord their power over others and claim divine channelship. It shouldn't be necessary, for each person has a pathway open to Jehovah. This is where two classes don't make sense - either Christ died for all or Christ died for 144,000. Which sounds like divine love to you?
