When, if ever, are dried herbs preferable to fresh herbs?

Dry rubs are one case that I can think of where dry is specifically necessary, so you can grind them up properly.

The main advantage to dry herbs is that they're available year round. When you're dealing with winter dishes, dry herbs would've been the norm to have used at that time.

If you are going to substitute, you'll need to add more (typically about 3x as much, as the dry is usually more concentrated), and you'll want to add it late in the cooking process, while dry herbs are usually added early.

One exception to the rule is bay leaves -- you'll still need to add them early, and you'll want to reduce the number.

Certain herbs are very mild when fresh and do not develop their full smell and flavour until dried; e.g. bayleaf, oregano.

Fresh herbs generally have short storage times. When substituting you typically need to add much more of the herb, as drying shrinks it concentrating the flavour.

I tend to go fresh whenever possible.

Some useful tips:

• When using dried, crush them first. I typically smash them with my thumb a few times into the palm of my other hand. This helps release the essential oils in the dried herbs.
• When using fresh in place of dried use slightly more. I'm not an herb-measurer, I eyeball, but I always use roughly 25% more. The fresh herbs tend to have a fresher, yet milder flavor. They also give up their essential oils much easier than dried, so the oils can evaporate and cook away quicker.
• When using fresh in place of dried, add later in the cooking process if possible. This depends a lot on which herbs in particular. The reason behind this is similar to the previous tip, fresh herbs are tenderer and can do a flavor dump very early in the process.

There is no one rule that holds all when it comes to dry/fresh herbs. There is so much variation between herbs in how they respond to the drying process. Basil, for instance, loses most of it intense flavor in the drying process. As pointed out to some others most kinds of oregano have a very different flavour when dried. Rosemary on the other hand maintains its flavour very well when dried. My advice is to make the decision on a herb by herb basis.

The above refers to regular drying. Note that freeze drying often maintains flavours of the fresh herb more. It’s however not common to find freeze dried herbs in regular shops.

Heating herbs can discolor them - if you've seen someone put fresh basil on a pizza before baking it, you can observe what happens. Same goes for sauces like tomato sauce that simmer for hours. If you want the flavor to infuse into the dish you will need to use dried. In addition, fresh herb flavor is more delicate. If you need to supercharge a dish but don't want to change the color, use dried herbs.

Dry rubs are a great use for dried herbs, but fresh can be pureed to a paste and used under the skin, so they can be useful for outer seasoning, too.

Choosing one or the other can depend on how long a dish is cooking. Dried herbs take a while to impart flavor so they're mostly useful for longer cooking dishes. Also, getting a piece of dried herb that hasn't had time to hydrate & soften can be really offputting to eat! Often, however, I use both- dried or sturdier herbs at the beginning & fresher/more tender herbs at the end when the dish is pulled off the heat. That way you get layers of flavor, not just top notes.