The story goes that my Mom had gotten indigestion from pre natal vitamins and was pretty fed up in general when pregnant with me. She tossed the vitamins out the front window into a planted bed and the climbing roses made it up the trellis onto the roof with prolific and large blooms. My Godfather claims they were tinged with additional color and never grew as well in following years. As for my opinion? take the vitamins yourself and pee on the compost. I don’t know what the “obvious comment” is and I don’t know what you mean by the plants can’t use it, or it’s inefficient.
I live in one of the most, if not *the* most, intensely agricultured areas in the U.S (with a very great variety of both food and horticultural crops), and the farmers/nurserymen, who are not known to spend unneccessarily, are quite intensive users of micronutrient sprays, the main one available locally being KeyPlex. I take my cue for home gardening from them, use it on everything, and have great health and vigor throughout the garden. Not only that, but the plants are so naturally strong, that I have needed no pesticides whatever. Sure, the bugs chew things, but ultimate damage is insignificant.
With vitamins you’re dealing with dollars per milligrams such that even a small box of expensive Miracle Grow will be hundreds of times cheaper. IOW, use of multi-vitamins from your typical $8.00 container might be a kind (although extravagant) gesture for a house plant but is far less likely to revolutionize agriculture 
I didn’t say they were human vitamins. I said if you want to give vitamins to plants, the way to do it is with micronutrient foliar sprays. It goes without saying that such would be formulated for the nutritional needs of plants. We do happen to have intractible soil problems in my region. In the words of one soil analyst “the only thing we don’t need is calcium”.
