A downside with this build is that if you have thousands of energy sigils (2k+) and good gear to get you 4-5 50 point dps skills then you will run out of energy quite fast. You only get a few strikes in between cd on skills anyhow and you will need to take a 5 second break every 2-10 rounds of spell cd (depends on how many sigils u have).Fire mages get dex/focus because the best DPS build for a fire mage used to be (used to be meaning pre necro loot...probably still is best dps build actually but I have no necro loot to test with) H2H with no mainhand and an offhand 150 fire axe.
In any case dex dosent help with dps just landing hits which you could use an attack pot for. Same goes with druids who actually use their totems.