Lonely Planet is an Australian creation, so it might be odd that I am bagging it, but...
The original cycling guides for Australia and I think NZ were written by a cycling journalist who was a very strong rider. I met him when he covered Tasmania. An experienced cycle-touring friend and I looked at the result when Lonely Planet's Australian guide came out, and frankly, the distance estimations and times were very ambitious for anyone but the guy's riding ability, judging by what we saw for Tasmania. Others have confirmed what we thought. That seemed to be the major weakness, and I am not sure if the same didn't apply to the NZ guide as well.
I also gain the impression that fancier is not necessarily better in terms of information, and that Lonely Planet may well be outgrowing itself. Hence the rise of Rough Guide as an alternative.
Having said that, I have never used any of these guides (although I do listen carefully to media profiles of founders of things like Lonely Planet and where they are now), and instead rely on information on the ground as I go, or what I can get from the internet.
Eric's observation about MTV gives me an uneasy feeling as well. Not so much for the information contained in the guides, but the pressure that results from increased numbers of tourists going to more isolated destinations because they are linked to trendy TV audiences.
A TV program I saw at the weekend was the epitome of Western crassness in tourism that included serious invasion of privacy in a remote North African village. Of course, the presenter thought it all hale and hearty with his conflicted information, so young travellers in particular are likely to think it's OK to do what he did.
That's the human aspect. I've seen the result of the environmental aspect on my home patch, too, where the influx of trendy walkers has resulted in severe pressures on tracks and surrounding wilderness areas.