To what effect? You can have a terrain and weather active at the same time. I've seen one of the NPC's in X/Y's Battle Chateau have her Florges use Misty Terrain and her Goodra use Rain Dance after, and have beaten her several times while both are still in effect.
Mainly for synergy purposes. I think that some of the terrains cna work well with the weather for or aganst certain types, like a Superchared always accurate Thunder, or am instant solarbeam with recovery. One combination can hinder dragons to an extent (Hail+Misty terrain).