I think the answer is pretty clear by now - you can't go wrong with self-written KnowledgeBase
I can also see, why many people prefer to purchase a ready-list and then customize. For starters some important subjects can easily slip your mind, so having even the topics only is helpful enough. From there on you'll need to start modifying, excluding irrelevant articles and generally "pimp your KB"
. The content does make a pretty big difference for SEO, especially with the new updates, so make sure to style this section in a unique, yet easy-to-understand fashion. One advice if you decide to delegate this task to your employees - choose carefully and choose only 1 person if possible - the more people that handle this task, the more different writing styles, terminology and techniques there will be and this can lead to a chaotic and jumbled KB
However I don't believe it is very effective to incorporate the KB with the blog - I see the blog more as the company image and it should primarily be used to tell your clients what's going on with you - procedures, events, innovations... Sure, you can also include a doc part for further help, but a separate KB is essential.
As for its usefulness, you don't need to worry so much if its widely used or not - many people will still prefer to simply open a live chat or pick up the phone and get an immediate response instead bothering to read, but this is a subject of a completely different occurance - bottom line is that you need to provide the written guuides on your websites, regardless if they are often used or not.
At the ened of the day, its a great feeling to look at this section and know that it's your offspring, it really gives an extra satisfaction, believe me