Like BC, in the first tier of Wrath gear, we'll be close enough to others to be competitive.
With this change, we won't be the highest dps casters, but still in there with the others.
And bring a fair bit of utility to our raids, two shammies in 25-man's or one shammy in 10-man's.

The best part is built in scaling. In BC, somewhere mid T5 we slipped from upper caster dps to middle-lower caster dps.
It looked like this would repeat in Wrath.
With a scaling talent, we should remain competitive at any level of gear.

And if we don't, it's very easy for Blizzard to adjust our scaling talent rather then the base spell.
Adjusting the scaling keeps us competitive at all levels of gear.
Adjusting the base spell overpowers us at lower levels of gear and underpowers us at higher levels.