Yeast are not bacteria. They are fungi. It takes both yeast and lactobacilli to make a sourdough starter. You don't have to worry about the bacilli. Start a starter with yeast and they will come.
Flour and water do not spontaneously become sourdough without yeast. Some sourdough yeasts are airborne and local. Some got into the flour at the millers and are packed into the bag by the time you get it.
The addition of grapes to a starter's starter is an attempt to get the yeast from the grape skins into the starter; and/or to feed the sourdough yeast spores as they reproduce.
Some of the things people do in the kitchen aren't well thought out. They do them because they work, or because they worked once, or because they worked once for someone's grandmother, or they worked for some schmo who wrote a cook book, or worked for her grandmother, or because someone else told the schmo they would work, or...
There are sourdough yeasts, good sourdough yeasts, and great sourdough yeasts. Since you can acquire a great one for free, it doesn't make sense (at least not to me) to take a chance on what might come in the bag of flour or through the window. Nevertheless, you KNOW I'd try it wherever I moved. Venice - good sourdough. Hollywood - OK sourdough. Monrovia - really lousy. Who knows? Maybe it's the lactobacilli.
You can usually get a good starter going by making a poolish with conventional yeast and just keeping the little SOB going. After even four or five days on the counter it gets nicely sour. I'm not sure if this has something to do with local yeast getting in the poolish and taking it over, or it just happens when you give commercial yeast a chance to stretch its wings. Like sending a kid to college, you know that whatever comes back with its laundry won't bear a lot of resemblance to what left your house with a suitcase full of clean clothes. Sic transit.
If you don't use a starter to start your starter, you not only stand the chance of getting one which doesn't taste particularly good or doesn't pack enough rise, you run the risk of a more robust, local yeast taking a sponge over.
I've killed a few starters, and watched others die out for no apparent reason, and even had one linger like Camille, getting weaker for months before I got bored and trashed it. But I never lived with one which went bad in the way Ross describes. I believe it though. Under the best of circumstances keeping a starter can be interesting in the same sense the word is used in the Chinese curse.
Anyone you know who bakes and keeps a sourdough starter -- and by keep, I mean like a zoo keeper -- will almost certainly be happy to share some live starter with you. We are a very nice group of people. Or, misery loves company. We bake. You decide.
If you want to get a great starter but otherwise begin from scratch, buy yourself a couple of envelopes and a couple of stamps, navigate your browser to Carl's Friends, click on the appropriate link and follow the directions. Other than the stamped envelopes, it's free. Download the recipe booklet while you're at the site. It's a good start.
BDL
