Yes, I'm laughing at the factory comparison too .
My dd is in day nursery for 8 -ish hours a day, she has free play, structured activities, trips to the park and library (depending on the weather), lunch, nap, more play or activities, sometimes another trip out and then some more play. I'm sure it is all very exciting for her - she certainly seems excited when I take her, but I'm not sure how that is more stimulation than the days when I am at home with her, when we do things like shopping, swimming, walks to the park, activities, she plays with her toys etc. Perhaps she should be lying in a darkened room to recover...
But to the OP, no, I wouldn't leave her for 15 hours a day - she goes to nursery because I have to go to work, otherwise she's at home with me. But that said, if her nursery did operate a longer service, then I'd be happier to leave her there than with a babysitter who didn't know her and what she liked, if I had to be somewhere else (which I don't because I haven't been out in about two and a half years now....)