There are numbers here to be found if you look for them. Also, many more numbers over at ElitistJerks.
But long story short: More Damage = More threat and Faster Boss kills. Plus it's a pretty essential AoE tanking ability.
Though, if you find you need the points elsewhere you can easily drop it. It won't make or break you at all as a tank. What you end up speccing into totally depends on what your personal raid needs.
Hope that helps.
"A man can learn twice as much from milking a cat as two men can from a very tall fourth man.
- Mark "Alec Baldwin" Twain