By default you can shock once every 6 seconds.

You can reduce the recast delay by 1 second, via the 5 points in Reverbation.
And by 2 seconds, via 2 points in Booming Echoes.

If the target has Flame Shock on it, the Lava Burst hits a lot harder.

But if you Flame Shock the first toon and then prepare to Lava Burst.
The priest is watching for the nukes to Pain Suppression.
The paladin is watching for the nukes to Blessing of Protection.
Etc.

But if you can Flame Shock one toon, and then 3 seconds later Flame Shock another.
That's a lot better then 6 seconds between Flame Shocks.

You'll also have a macro like this one...
/castsequence [target=Focus] Elemental Mastery, Flame Shock, Lava Burst, Chain Lightning

So you can Flame Shock and then virtually instantly follow up with a Lava Burst.
If you had just Flame Shocked one target, it would be nice to be able to follow up with this kill macro in 3 seconds, then need to wait for 6 seconds.

For pvp, you need the "Glyph of Shocking" to reduce the GCD on shock spells too.
That's a glyph that is pretty useless in PvE, or even BG's, but is extremely powerful in arena.