I find LinkedIn reasonably useful as a professional Rolodex. I've never fleshed out a resume on it and I mostly just ignore emails that come through it. But, then, I haven't gotten a job other than directly through personal contacts in decades.
ADDED: To your broader points, a resume (especially for experienced people) has long been one of those things that you need to have and that people may use to roughly gauge potential fit. But very few people are making decisions based on only a resume, especially for senior positions. And portfolios, in whatever form, have always tended to trump resumes in certain fields. (Which can admittedly be problematic in software because some have portfolios they can share and other, potentially equally skilled developers, do not.)
I use it in the same Rolodex fashion. I feel it also adds weight to a resume. If someone can find connections of actual people at other companies that I used to work with, it validates my resume, at least at a high-level.
I've received countless job opportunities through LinkedIn. My current position, I reached out to a former colleague to join that company via LinkedIn. It worked well.
ADDED: To your broader points, a resume (especially for experienced people) has long been one of those things that you need to have and that people may use to roughly gauge potential fit. But very few people are making decisions based on only a resume, especially for senior positions. And portfolios, in whatever form, have always tended to trump resumes in certain fields. (Which can admittedly be problematic in software because some have portfolios they can share and other, potentially equally skilled developers, do not.)