I don’t understand the value of tags. If you OCR all your pdfs, and Spotlight indexes every word of every file on your computer, what is the purpose or need for tags?
One could say the same thing about folders or colored labels. It’s possible to find anything via search or exhaustive browsing, but in many cases other means of organization and annotation are helpful. Consider, for example, that there is no way to derive a file’s unread or flagged status from the words that it contains; these relate to what you have done and how important you currently find the file.