I would definitely concur that LinkedIn is worth it to join, in terms of both connecting to business-related materials, online resume posting services, and networking opportunities in whatever business area one would happen to be in. Both my current job and my previous job were the result of those connecting with me through LinkedIn.
As far as quality of the discussion is concerned, that would be field-dependent. For those in STEM fields like myself (for those who don't know the acronym, it stands for science, technology, engineering, mathematics) who wish to engage in technical discussions, LinkedIn is actually quite good, particularly in Group discussions which you have the option of joining. I agree though that there really is not a good mechanism to engage in debate.