I've met a guidebook writer or three over the years and the truth is that they vary. The best are either specialists in a particular region (e.g. Hilary Bradt in parts of Africa) or young enough to see the job as exciting despite the lousy pay. No guidebook series is uniformly great. I tend to like the Footprints and Moon Handbooks ones, but they don't cover everywhere. I'm not a big fan of Lonely Planet for the most part (their maps are uniformly terrible and they have an annoying tendency to recommend American fast food chains for cheap meals everywhere), but I've used them when nothing else is available. And some of their books are fine, e.g. the pocket city guides (though they still tend to have inadequate maps).
You also always have to remember that things change faster than books are published. Restaurants are especially prone to come and go, but a decent hotel can be in the throes of renovation or a great new hostel can open up or a bit of civil unrest could keep you from getting where you were going in the first place. There's no way to be prepared for everything.
It also matters that I am not always looking for the same thing in a guidebook. Depending on the destination, I could be looking for hotel recommendations or details on how to get between places or cultural background or ideas on things to see. For example, Frommer's Japan was fairly disappointing in many ways but led me to a great cheap hotel in Osaka and was the reason I made an excursion to Kurashiki, one of my favorite places in Japan. And I could fill in some of its gaps via local tourist offices (a source of great maps and useful advice in almost every place that actually has a tourist office).
Bottom line is that there are lots of reasons not to rely solely on guidebooks and this scandal is just another on a long list.