Not on a medicinal level but on a fragrance blend level I do the same thing, I devise 'recipes' for bath oil blends or pot pourri and instinctively know what oils need to be added, I often don't follow recipes to the letter either as intuition tells me that a certain oil would be better for the mix than the one recommended.
I have come across herbs that were needed medicinally both for myself and friends, one friend had really bad ezcema and was ignoring the chickweed that had sprung up in her lawn and borders, I pointed out the benefits of chickweed cream for soothing eczema and gave her a DIY recipe from one of my books, she harvested the chickweed made the cream and after months of sore itchy skin her eczema calmed down.
In my own garden I got irritated by the way that Herb Bennet or Wood Avens kept springing up everywhere, I've strived to erradicate it before now but it kept worming its way back into the borders and paths. This year I gave up the battle and decided to let what wishes to grow in my garden grow, considering if it has no use I'll weed it out at the time if its stopping something else growing that I do need. And if it comes back again I'll see if I need it upon its return. This year the wood avens sprung up once again, so I investigated and being a keen herb beer and wine maker imagine my surprise when I read the following in Mrs Grieve's Modern Herbal "Augsburg Ale is said to owe its peculiar flavour to the addition of a small bag of Avens in each cask. The fresh root imparts a pleasant clove-like flavour to the liquor, preserves it from turning sour, and adds to its wholesome properties. " Okay I don't 'need' the wood avens, but I can certainly make good use of it!
I do believe that mother nature gives us herbs that we can use to treat ourselves right on our own doorsteps, although this makes me wonder why? In some areas large amounts of a certain wild herb may grow, but it doesn't mean that everyone in the area suffers from a condition that herb is required to ease or cure. Granted local conditions, soil type etc all play their part in regional gluts of herbs, but you can't help but wonder why certain things that haven't grown in your garden or area before turn up when needed?
Debs