Lately fantasy series have gotten a bit of a bad reputation. Too many writers drag them out, volume after volume with no resolution in sight, until the audience gives up. Maybe that's why trilogies are so popular. With a trilogy, you at least know an ending is going to happen.
Me, I've always felt the strongest series form is five books. There's plenty of room to deepen characters and explore themes, but the ending still happens before the audience has time to get board and leave. Best of all, there's less chance of falling into `middle book' syndrome, where the middle book feels as if it only exists to hold the beginning and ending apart.
The strength of the series is its interconnected nature. In the Prydain Chronicles, some of the individual books are a little weak. In book one, for example, the hero is knocked out for the climax. Later, as the books build on each other, you realize that this ending is actually good in the thematic sense. Taran has to be humbled before he can become a hero. If he'd been part of that early battle his pride would have been harder to overcome. The theme of heroism through humility is examined over and over in ways that would never be possible if Lloyd Alexander had only written a single volume.
A series also allows for amazing plot twists because the author can lay the groundwork for them through several volumes. Emily Rodda's Deltora Quest books are a lovely example of this. She slowly introduces allies and possible enemies over several volumes. Actions constantly need reinterpreted from book to book. Her main characters go from almost strangers to a close-knit team in a very believable fashion because you can see the gradual change as it happens. This is another case where one book just wouldn't have the depth that a series is able to display.
A series is a tricky thing. The length means there's more room for mistakes, but with careful handling, the series gives a sense of depth that no single volume will ever be able to match.