Really? If I had a horse who looked like that when they weren't sweaty I'd be thinking cushings and all sorts of things. I can see sweat n the neck, girth, flank/stifle, and even a bit on the loins. Sweat on the loins and girth are bad news in winter as they are close to vital organs, and a chill in these places like you get with sweat can cause pnuemonia.
If I had a horse that was simply a pasture pet or wasn't ridden hard enough to sweat, I wouldn't rug. Horses equip themselves very efficiently for winter without our help.
BUT, we have changed their use and environment and must adjust our care appropriately. Any of my horses who work hard enough in winter to bring up a sweat, are rugged. Simple as that. It is a health issue and I won't risk my horses getting pnuemonia or losing condition.