I make sourdough bread, and I use a mother. The idea behind lactobacillus is to give it that distinctive "sour" flavor. it's really good in bread and pizza dough, but if you've ever had a lacto infection in your beer, you'll recognize it. It's sour! Think yogurt, sauerkrat, kim chee, cider. Lactobacilli, especially L. casei and L. brevis, are some of the most common beer spoilage organisms.
So, that's what the others are trying to say. Not a true vinegar (that's aceterobacter contamination) but close enough to be unpalatable in a beer. Well, that's not entirely true- in Belgium, I think some "wild" organisms are actually prized and beer is fermented in open vats. I'm no expert on Belgians (I don't care for them, particularly lambics) but I think Brewtopia (?) is and could give more thoughts on this aspect.