When someone is almost dead, the little bit extra can kill them.

If you're not close to killing someone, due to pillar humping... or they're dead easily for your gear vs theirs, its not a huge benefit.

You can macro it into your "big button", and have the use automatic, when you do the elemental mastery kill them macro.

I'm in the process of leveling teams, specifically for battleground pvp's, and both sets will have 5x Engineers.
Lots of other toys are fun.
Also look at things like the Darkmoon Card, for crit and shadow damage, etc.