Those would be great for cats, especially for going to the vet, but I'm not a fan of carrying dogs around no matter what. They lose a lot of their ability to be actual dogs when people start carrying them around EVERYWHERE and their feet never touch the ground.
Now if you're going to be a LOT of walking, fine, put them in something--AFTER they get tired! But to continually deprive them of the ability to follow scent like real dogs do? No. Just. No.