Man, as a project and product manager I gotta tell you that I completely agree with the cons you listed. Nowadays, I invest less and less time and effort on those because things tend to be so dynamic that, unless you update your plan every week or so, it is going to be outdated quickly.

Sometimes I find myself reporting to some very pragmatic and old-fashioned stakeholders that have a need for extensive paperwork and then I just have to sit and write a very robust business plan. Sometimes I really go crazy on them just for the hell of it. I actually had to do that today so I spent a good part of the afternoon drafting an excessively detailed business plan lol