Why is this desired in western pleasure?

We most likely differ on a matter of degree to which the back in rounded.

https://www.youtube.com/watch?v=FB5pUYrfRgk

This horse is truly wonderful. I can find much more horses like the other link I posted. They're everywhere.

As to regards of the other video, that was just disgusting. If somebody found that at a show they'd be DQed without a doubt. I love how the video owner deleted it, LOL. Just goes to show that they knew what they did was wrong.

Here is another one...This horse is A Certain Vino, he won the All American Quarter Horse Congress 2012 Sr. Western Pleasure, and one more class that I cannot remember....But all I do remember is that he won a trailer, a blue ribbon show saddle, and some other things...He's the kind of horse that just goes into the show pen and everybody knows he's going to win...A true natural.

https://www.youtube.com/watch?v=yOGV...feature=relmfu

Its not like they're loping around the arena looking like this...

https://www.youtube.com/watch?v=sS2ecubFbAE

Slightly OT...
No Doubt I'm Lazy (I think that's his name anyway!) the first video you posted.
I absolutely love him. His movement is gorgeous .
If only if only...

No Doubt I'm Lazy is even better than his dad Lazy Loper.

Thats for sure. I think he could have been a cute (very small) equitation horse. He definitely had a ground-covering stride.

I agree. I'd heard of Lazy Loper before, wasn't a fan, but after seeing No Doubt I'm Lazy, I changed my mind! Amazing horse.

That head set is against AQHA rules.

Which video?