Agree with you there Paul, which is why I drop mine quite a lot for techy trails. It's all about being able to move about dynamically – all the time 🙂
I would get one if doing the Mega again, as it was about the only thing my bike lacked this year. Really frustrating to get overtaken by folk, who've just clicked their saddles down at the start of a big descent, having overtaken them on the pedalling section.
And as for this 'get yourself right over the back of the saddle' stuff….that just makes the bike handle like a pig, under-steering badly, totally unweights the front end, hinders braking and balance. Saddle at optimum pedalling height is great for XC, but try to land a decent-sized jump with a seatpost at max extension and you'll not be able to absorb the landing very well (if at all) without being bashed by the saddle.