One example of the WTS's vacillation on this subject is "guardian angels." Until 1974 the teaching was that angels would actually provide physical protection for true worshippers; in 1974 that was changed to mean only 'spiritual' protection.
This is a hard theory to maintain, and gets harder every time a perfect candidate for angelic protection ends up dead. Bad things DO happen to faithful witnesses, even extremely vital ones, even while in some form of service. Remember last year when a small plane carrying 5 Regional Building Committee overseers crashed, killing all of them? The plane was traveling to quick build project meeting. Where were THEIR guardian angels? [inkling]