A lot of the time, they feel a bit unnecessary; one of the weaker parts of the narrative they're trying to enhance. Usually, it's the attempt at adding depth by using a combination of psychoanalytic metaphor and (more often than not) prophetic foresight which seems to fall a bit flat (with cunningly crafted exceptions, of course – take Twin Peaks, for example). As if, while attempting to add subtlety and depth, the writer has instead ended up making their narrative a bit obvious and shallow, or far too obscure to interpret.
All of this being said, I am quite fond of dream worlds. It's a niche belonging to portal fantasy, in which the portal is the simple act of falling asleep, and it has a history of producing classics. Alice in Wonderland, The Wizard of Oz (film!), and even more contemporary essential pieces of reading, like Neil Gaiman's Sandman, have their own dedicated realms of dreaming, and each is considered important.