I have a min pin so I can explain alittle as to why they should not be left outside unattended:
#1 - they are hunters. Actually ratters. So any small movement and they are off to hunt down prey...even if it's not prey.
#2 - because they are hunters, they are very quick. They will borrow in high brush or even go into small holes etc. Once they are gone, they are very difficult to catch unless you have a good recall on your dog.
#3- they are so small that other animals can view them as prey. Wild animals and even some large birds can kill these dogs. Min pins are also way too small to bring to dog parks. They can be easily hurt by other larger dogs.
#4 - min pins are tough characters. They are bold and confrontational (not a rule however). This can cause problems with other dogs and again could result in a dog getting hurt..and more than likely the small one.
#5 - min pins are escape artitis and can squeeze themselves through the tinest of areas. I know as mine squeezed under our fence and took off. Luckily I got her back and how she got out will never be fixed as she dug the tinest hole and underground and under the fence. Na - they are houdinis!
As this dog you are considering is older and has an eye issue as well as nervous...I am certain you would not consider leaving this dog unattended.