well I was just wondering if there is any reason to take the fear perk when I already have the calm perk ?

You can keep hitting Fear affected enemies and they won't fight back, they just keep running away.

Calm enemies will start fighting again as soon as you hit them.

that dosnt seem very practical if you have to chase them around

Yeah, that's why I don't usually like Fear effects, but it's nice to get some enemies out of the fight so you can deal with the others one at a time.

Or just using the Werewolf roar to cause mass fear in an entire city with no cost at all.

If you fear an enemy in a dungeon they usually go and cower In a corner. It's actually pretty cool. Much better than world of warcraftd fear effect where they just run around

need to combine fear with a slow or stamina drain.

If your fleeing enemy hits zero stamina, it'll just fall over trembling on the floor. Similar results as paralyze, but this one you can do at low level.

Fear isn't run till the duration up fear. It seems to be run a sort distance and try to keep that distance from the spell caster. I find it to be a lot better than calm in most cases, though it occasionally adds in extra fun as they run to a place with friends.