I fostered a full on kelpie recently. I had her for a couple of months before a suitable home was found. I was quite happy to see her off to her forever home. I have enough dogs of my own to have wanted to keep her. I invested some time in training her because she was so feral, but was careful not to get attached - yes a bit like minding a friends dogs.
However I have my own dogs and no wish to add to them, just to help the occassional working dog in need.