Spokescycles wrote, "I'm also wholly unconvinced at the need to spend £200 more than a QR when it's not inconvenient unless you ride nonstop or your mates are mindblowingly impatient."
If you're just using them as a QR replacement then you're right, they're fairly pointless. Places like Glentress, frinstance, there's really only 1 place on the entire red trail where it's an advantage, because almost every section is either seat up or seat down- you just stop at the start.
But, go to Kirroughtree, as a nice benchmark, and do Talnotry Hill- it's a big climb followed by an immense descent/traverse, which is pedally in places and has some decent size rock features in others. You either need to compromise, or stop and start constantly, or have a dropper seatpost. Most people compromise of course. But that's where they stop doing the same job as a QR.
I think some people see them as some sort of "trail centre rider" accessory but I think the opposite's more true, they're a brilliant XC tool- just riding along, see something interesting, ride it, then just riding along again without so much as a pause. They're generally more use in the real world than they are at big-up-big-down trail centres.