Maybe it's me, but the reasoning behind having the home page example be it's own task vs a section is for feedback. It depends on how many people are weighing in on each task, but I don't see how breaking it down by header, footer, main mockup, is very conducive to feedback.
I run into this issue a lot - my role spans managing design, video production, events, etc. and it can sometimes become confusing if you break it down into tasks like that, whereas if I have a 'Mockup Home Page' task, then I create the small 'milestone' subtasks if needed (like, creative brief, etc.) - then the actual feedback back and forth is done on the main task.
If we used sections (and trust me I use them liberally in my projects, I think it would be confusing to casual users as to where to add their feedback. For instance, I'm looking at the header task, and I think about how it needs to also reflect the footer - well now I sent feedback for something that's not the footer task, etc. and it could get confusing if multiple people are doing it, unless you have very strict plans - so if feedback or comments overlap tasks, copy them to both, etc.
I don't know, maybe these are problems that have different solutions, but it's why I find subtasks useful despite the drawbacks.
I do agree on a lot of the other points made here about them though.