Fear is not a good protection mechanism.
Sure its very powerful against those unprepared to deal with it.
But its on diminishing returns, and there are so many counters to it.

At the same time, as a team of players you need some kind of mechanic to deal with fear.
Shammies can drop Tremor totems to help mitigate this.
If you have a priest in your team, you can Fear Ward the priest and if you're quick Mass Dispel Magic your team, when fear bombed.

Anything you want to play can work in battlegrounds, if you plan it out and can coordinate it well.
5x Death Knights can be quite powerful. Blood will help top them off to a large degree.
A dedicated healer can make the team that much harder to deal with.

Arena is a totally different ball park.
Depends how highly rated you wish to go.
The more competitive play demands specific compositions.
Lots of info in the wiki and on these forums.