I think it's a bit of a broad statement to suggest Project Managers (in isolation) treat the TAGs a tick box exercise, and wrong to say they leave it till the end.  Our practices (at least the project managers that follow them) are to not only get the right input up front (from a project perspective ensuring the right technical direction is given reduces risk) but to continue to ask whether review is needed throughout the project.   From my perspective, it's typically other disciplines that don't want to continue to return to the TAG forum for further discussion.  I've only seen a couple of examples of projects going in towards the end of their projects, and they've been suitably admonished for it.