Eviolite is mainly used on a pre-evo that has similar defenses in one stat, but inferioir in another, to take those hits better than its evolution. For instance, Eviolite Chansey is way better than Blissey at taking Physical hits. The choice between the two is if you want more bulk on one side(Chansey), or a viable use of Flamethrower or Ice Beam (Blissey). Same goes for Specially Defensive Gligar. Eviolite boosts its nice defense, while allowing you to invest in a Specially Defensive spread that Gliscor can't quite run with his SpD. However, most pre-evos have less HP than their elder form, so it's all a matter of opinion.
Edit: Eviolite Dusclops >>>>>>> Eviolite Misdreavus, tbh.