That same situation happened on an episode of Law & Order SVU. The dog walker hired students and when the owners went out of town, the student/walker had a party and was killed. Ok, so that's totally just a show, but I had to mention it.
I work from 10 am to 6 pm everyday. At first I did what you did, coming home for a half hour, but it became too much. I finally got Riley a sister, Lily, to keep him company. I let them roam around the house and they are fine. I actually just got a webcam, but it's not setup right yet. I can't wait to be able to see what they do all day when I'm at work. I think it's safer to just keep them at home then to put them in the hands of someone else, unless you really knew the person. I think sometimes we forget that are small little babies are still dogs, and dogs stay home alone all the time...otherwise no one would be able to have a dog. My boss let me bring in Lily the other day and it was awesome, but it was only for the day.