J. Kenji López-Alt, the Managing Culinary Director of Serious Eats, and author of the James Beard Award-nominated column, and now book, The Food Lab, comparison cooked using fresh and dried herbs. He found, to his surprise, that the flavor difference between fresh and dried "Mediterranean" or dry climate herbs, once cooked to release the flavors, was not significant. Why? López-Alt explains in his Serious Eats column.
Many chefs assert that fresh herbs are superior to dried herbs, and they're right—most of the time. Most herbs contain flavor compounds that are more volatile than water, which means that the drying process that removes water also ends up removing flavor.
But it's not always the case, and here's why: Savory herbs that tend to grow in hot, relatively dry climates— like oregano, for instance—have flavor compounds that are stable at high temperatures and are well ￼￼￼￼￼￼￼contained within the leaf. They have to be, in order to withstand the high temperatures and lack of humidity in their natural environment. With these dried herbs, as long as you cook them for long enough to soften them, the flavor is just as good as with fresh—and they're a whole lot cheaper and more convenient to use.
Oregano, Rosemary, Thyme and Bay Leaf are herbs frequently used in Spanish cooking, and I find that these jarred herbs and the Spanish spice blends containing them seem to retain their robust flavors beyond the typical 6 months. Now I understand why.