Weeeell.... they are very worth it, except, of course, for when they aren't.
Mostly, the tier builds are just about giving you extra options for how to build your army, rather than giving you any sort of boost, and indeed most of them have more weaknesses because of their restrictions than strengths due to the bonuses. However, some tier lists are quite competitive, and very fun to play. I'm not sure how Elyliths list plays, but if it looks good to you, perhaps it's reasonably strong.
Still, I usually wouldn't recommend aiming for a tier list until you had enough models for a general mix-n-match army, especially if you're a newer player. The restrictions on what you can take, and often having to take multiples of units that won't work well with other casters can really limit you if your just starting to collect. That said, if you only have to swap out a couple of models to make your general list into a tier list, then I say go for it.
Last edited by maddermax; 05-31-11 at 02:41 AM.